When does identical twinning happen?

When does identical twinning happen?

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What is the window during which division into twins is possible (for instance can it still happen after implantation in the uterus)? Is one twin the "original" and the other its clone?

In the case of two healthy, viable identical twins, the separation generally occurs at the first division from one zygote to two cells. So they are equal partners in the split, not an original and a clone. During the first several cell divisions in the morula and blastula, there is no significant growth. So these divisions produce smaller and smaller cells. If one of these smaller cells is separated, it is very unlikely to be substantial enough to survive and implant in the uterus.

This page describes what happens pretty well:

Division into twins can apparently happen at several stages up to and including implantation, but not much after that. The later the division the worse the prognosis because the more the twins share. The article mentions three stages in particular: division at the 2-blastomere stage (i.e. when the original zygote is cleaving), which leads to two completely separate embryos that implant separately, division at the embryoblast stage which leads to the embryos sharing a chorionic cavity (i.e. placenta), and division at the embryonic disk stage, which happens after implantation and involves the fetuses sharing everything - apparently to the point that they rarely survive until birth because they get tangled in each other's umbilical cords. Later divisions, after the primitive streak starts forming (i.e. about two weeks after fertilization), can lead to conjoined twins, because the division happened when the cells have already started to differentiate into which part of the body they'll be.

In no case can one twin be considered "the original" and the other not. Twinning involves whatever stage we're at dividing into two; we can't talk about one being an offshoot of the other, anymore than if you pour cake batter into two molds instead of one you don't end up with an original cake and a copy, you just end up with two cakes. Whose batter happened to be in the same bowl until some point in the baking process.

Moreover the twinning events usually involve a symmetrical division, meaning we can't even talk of one twin having a claim to a higher percentage of the original embryonic material or whatever. An exception may be parasitic twins and other examples of asymmetrical twins, but I haven't been able to find a good source on what causes those - it's unclear whether it involves twins resulting from an unequal division, or whether later development made them unequal.

Dizygotic twinning

The tendency to conceive spontaneous dizygotic (DZ) twins is a complex trait with important contributions from both environmental factors and genetic disposition. Twins are relatively common and occur on average 13 times per 1000 maternities, though the twinning frequency varies over time and geographic location. This variation is mostly attributed to the differences in DZ twinning rate, since the monozygotic twinning rate is relatively constant. DZ twinning is in part under genetic control, with mothers of DZ twins reporting significantly more female family members with DZ twins than mothers of monozygotic twins. Maternal factors such as genetic history, advanced age and increased parity are known to increase the risk of DZ twins. Recent research confirmed that taller mothers and mothers with a high body mass index (30>) are at greater risk of DZ twinning. Seasonality, smoking, oral contraceptive use and folic acid show less convincing associations with twinning. Genetic analysis is beginning to identify genes contributing to the variation in twinning. Mutations in one of these genes (growth differentiation factor 9) are significantly more frequent in mothers of DZ twins. However, the mutations are rare and only account for a small part of the genetic contribution for twinning.

Twinning and Twinship

In explaining the biology behind twinning, it is important to point out the different types of twins that exist. Fraternal or dizygotic twins occur when a woman releases two eggs, instead of one, during ovulation, making both eggs available for fertilization. Fraternal twins, of course, share a common prenatal environment and normally the same birth date. Since fraternal twins come from both two different eggs and two different spermatozoa, the pair can be either female/female, male/male, or female/male. Identical or monozygotic twins on the other hand, must either be male/male or female/female. Identical twins occur when the fertilized egg, or morula, splits. Because identical twins come from both the same egg and sperm, they are exactly alike in their genetic makeup (3). There is speculation that a third type of twin exists, an "identical/fraternal." These twins are said to be a result of a split in the mother's egg after ovulation but prior to fertilization. In these cases, the twins share identical genes from their mother, but differ in genetical makeup because they came from different sperm and thus have differing genes from their father (1).

Today, testing for monozygoticity in twins can be easily accomplished with DNA testing (3). However, studies done earlier in the century help to explain the traits that identical twins, in effect natural clones, have in common. In some of the classic studies of twins, including one done by Dr.Horatio H. Newman, the following criteria were used in determining monozygoticity. In order to be diagnosed as monozygotic twins, the pair had to exhibit a striking similarity in appearance, so much so that the twins are or have often been mistaken for each other (5). Height, fingertip pattern count, and fingertip ridge count had to be similar in monzygotic twins (4). The pair had to be essentially identical in hair color, hair texture, and hair form, as well as eye color, eye pigment, and iris pattern. Their skin needed to be of the same complexion (unless one was changed by tanning) and their skin had to display the same amount of body hair distribution on the face, neck, and hands. More importantly, the pair had to have virtually the same facial features, types of teeth, irregularities in dentition, as well as similarities in the size and shape of the hands and fingers. With most of the identical twins in the test, the details in their palm and finger patterns showed that one hand of an individual in a monozygotic pair, was more similar to one of the other twin's hands than to the individual's own opposite hand. In other words, the pair exhibited stronger cross resemblance than internal resemblance in their finger and palm patterns (5).

There exists a peculiar phenomenon among identical twins commonly referred to as reversed asymmetry or mirror imaging. If division of the morula occurs between the tenth and thirteenth days of development, the twins are commonly found to be mirror images of each other (1). The twins usually demonstrate this reversed asymmetry in handedness (right or left handed), hair whorl patterns, dentition, palm and fingertip details. While one twin has hair whorls growing clockwise, the other might have counterclockwise hair whorls. In some cases, such twins have birthmarks one the same part of the body, the one on the left and the other on the right (5). This phenomenon holds true not only for identical twins, but all identical multiple births. There can be triplets, quadruplets, even quintuplets who appear to be mirror images of each other, as in the case of the Dionne quintuplets. (These five girls are the only known case of identical quintuplets. Other multiple births can be combinations of monozygotes and dizygotes, the most common being all multiple zygotes fertilized simultaneously) (7). Two of the five identical Dionne girls appeared to be the last to separate from the singular zygote, hence probably between the tenth and thirteenth days, because they, among all of the girls, have the highest presence of reversed asymmetry.

After the thirteenth day of fertilization, twins who begin to split often do not make the full division. The result is what is commonly known as Siamese or conjoined twins. The term "Siamese" twins comes from a famous pair of twin from Siam who were joined at the breastbone (1). Of course, all Siamese twins are monozygotic and, in fact, the existence of Siamese twins helped to prove the existence of monozygotic twins (5). Siamese twins may be joined physically at any part of the body including entire torsos, the top or side of the cranium, hips, and chests. Some cases include sharing of vital organs to the point that the two cannot be separated without harm or even death of one of the twins. In some extreme cases of conjoined twins, one twin is actually within the other in the form of a cyst (7). These cysts form tumors in the abdomen, liver, or brain and contain parts or sometimes all of another fetus. In three documented cases, infants were found to be carrying unborn siblings inside of them. Though Siamese twins are monozygotic, often there exists large dissimilarities between the two individuals in weight, height, and head shape and size. Why these differences occur in all twins is a question that has been studied thoroughly. In the case of Siamese twins, most of the differences would likely be due to conditions in their prenatal environment (5). In all twins, however, reasons for the variation of physical and psychological traits include genetics, prenatal environments, and post-natal environments.

No individual develops without both a basis of heredity and influential environmental factors. Though factors such as eye and hair color are considered purely based on genetics, other factors including intelligence, body weight, and muscularity are markedly influenced by both genetics and environmental factors. Fraternal twins are as separate in genetic origin as any siblings coming from the same parent and thus can be as similar or different as any brothers or sisters except for the fact that their similarity in age often leads them to appear more genetically similar (5). Since identical twins are genetically alike, the reasons for differences between them lie not in the realm of heredity, but in the varying environmental conditions each are exposed to.

In order to explain how factors in the prenatal environment can effect twins and other multiple births, the typical prenatal environment must first be examined. Different sex, dizygotic twins are almost always dichorionic--surrounded by separate chorionic membranes. However, not all dizygotic twins are dichorionic and, similarly, not all monozygotic twins are monochorionic. Also, dichorionic twins can either share a common placenta, or have separate placentas. Thus to be identical twins does not necessarily mean having to share a common placenta nor does it connote being enveloped in a shared chorion, although this is often the case (5).

The prenatal environment of identical twins, often leads to dissimilarities between the twins which can continue into adulthood. For twins who share a common placenta the differences in blood supply, and thus nutrients, to opposite ends of the placenta are sometimes a factor (7). Usually, however, the case with identical twins is that the vascular invasions of the placenta meet toward the center of the placenta and compete for this area. As a result of this competition in a small area, sometimes anatosomes--extensive fusions--of capillaries will appear (4). More rarely the fusions are made vein to vein or artery to artery. This can cause a serious imbalance in blood exchange which in turn produces prominent differences in surviving twins including variations in weight and size. These variations "frequently persist for life" and, beyond size and weight, the differences in blood flow can effect an individual's "vigor and general health" and may even effect one's intelligence quotient (5).

A specific disorder, called Twin to Twin Transfusion Syndrome, is related to this idea of disproportionate blood flow. Twin to Twin Transfusion Syndrome is a disease of the placenta affecting only identical twins who are monochorionic-diamniotic or monochorionic-monoamniotic. One of the twins, the recipient twin, receives too much blood and becomes larger in size. The extra blood that this fetus receives is very viscous and thus the baby's heart over exerts itself in trying to pump blood. When left untreated, eighty to one hundred percent of the recipient babies die of heart failure. The other twin, the donor twin, in contrast receives too little blood. This fetus has very little fluid in its amniotic sac and it is normally smaller. Like recipient twins, eighty to one hundred percent of donor twins will die from heart failure (caused this time by severe anemia) if nothing is done to help. For those who do survive, the recipient twin, though larger, usually fares worse than the donor twin because it is more tired (9).

In an earlier study of both fraternal and identical twins, the average pair differences in weight and height, among other characteristics, were determined. The average weight difference of identical twins of all ages was found to be 4.1 pounds, with one pair reaching a difference as high as the fifteen to eighteen pound range. In the case of the fraternal twins, the average weight difference was 10.0 pounds, just slightly lower than the average for all siblings, with one pair reaching a difference in the thirty three to thirty six pound range. The differences in weight can be attributed to both prenatal and postnatal environmental conditions in both types of twins as well as genetics in the fraternal pairs. The average height difference of the identical twins showed to be a bit smaller than the difference in fraternal twins. The identical twin group showed an average difference of 1.7 centimeters, where the fraternals averaged 4.4 centimeters different. Since significant height differences only occurred in fraternal twins, height is thought to be largely an effect of genetics (5).

Because identical twins are genetically exactly the same, these twins have special appeal to scientists searching for the cause of occurrences such as disease, dissimilarities in intelligence, and other physical and psychological characteristics. Both identical twins and fraternal twins have been used in various studies to determine whether the causes of certain characteristics are genetic or environmental. This so-called nature-nurture question still is covered in much mystery, but generalizations can be made on a number of traits (6).

Differences in physical characteristics such as height, size of head, hand size, and muscularity between twins proved to be small and thus these characteristics can all be said to be least effected by environment and most by genetics. The fact that weight and Binet IQ and Stanford Achievement Test intelligence scores differed in identical twins more than the above physical characteristics shows that intelligence is affected to a greater extent by environment. The largest differences between identical twins occurred in the areas of educational achievement and temperament. This finding shows that educational achievement and personality are even more effected by environment (5).

However, whether it be from similar brain wave patterns, a common growing environment, or some unknown gene, identical twins, reared together or apart, seem to have some striking similarities beyond the obvious of an uncanny resemblance (6). In one study done, identical twins appeared to be concordant, or phenotypically similar in a number of mixed categories. Regarding disease, the three most common incidents of concordance were measles, diabetes mellitus, and rickets showing ninety-five, eighty-four, and eighty-eight percents respectively. Also, according to the study, eighty-nine percent of identical twins with Down's syndrome, will share that disorder with their twin brother or sister. Regarding psychology and psychological disorders, the four most common disorders demonstrating concordance were feeblemindedness, schizophrenia, manic-depressive psychosis, and criminality showing ninety-four, eighty, seventy-seven, and sixty-eight percents respectively (6). Numerous studies show that the concordance rates for identical twins having schizophrenia are always three to four times higher than the rates in fraternal twins (2).. Also, interestingly, Smoking habits, coffee drinking, and alcohol drinking showed high levels of concordance with, respectively, ninety-nine, ninety-four, and 100 percents. All of the above disorders revealed to have lower concordance percentages (much lower in some cases) in the tests done on dizygotic twins (6).

Findings such as these are helpful in explaining some incredible findings about twins reared apart. Two twins, one from California and one from Germany who had been parted at birth both exhibited strange traits such as wearing rubber bands around the wrist, clipping one's moustache in a peculiar way, and sneezing loudly to get attention as well as more common characteristics such as temperament and tempo. Other separated twins shared common phobias and defects originally thought to come from one's growth environment. These results suggest that more psychological traits like leadership ability, imagination, and vulnerability to both stress and alienation may be markedly hereditary (7).

Besides the biologically colorful "human-interest" stories of strikingly similar separated twins, there are many other intriguing aspects of twinning. Until recently, mothers were all expected to gain under forty pounds during pregnancy, no matter if she was carrying a single or multiple pregnancy. Now, doctors and medical experts have come up with what they consider an "ideal twin outcome." Twins are supposed to have a slightly shorter gestation period lasting anywhere from thirty five to thirty eight weeks. Triplets are even less showing an average for ideal outcome at thirty-four to thirty-five weeks. Also, the mean birth weights attributed as ideal are 2500-2900 grams per infant. Mothers with ideal twin outcomes gained forty-five pounds on average and mothers of triplets with ideal outcomes gained fifty to fifty-five pounds. Mothers with non-ideal outcomes gained less than thirty-five pounds (8).

Furthermore, research shows that in industrialized countries, the rate of mothers having multiple births is rising. This is most likely due to the fact that mothers are waiting longer for pregnancy and widespread use of technology to help reproduction. There are today one hundred and twenty five million living multiples in the world. In the industrialized US, there were 106,689 multiples born in 1996, 100,750 of them being twins, 5,298 triplets, 560 quadruplets, and 81 higher level pregnancies (8). The fact that mothers wait until a later age before having children makes the incidence of multiple birth even more high risk. All multiple pregnancies are high risk compared to singleton pregnancies and risks to the fetuses and the mother increase with the number of infants the mother is carrying. The risks include an increased occurrence of pre-term labor and low birth weights (which both are already less than singleton births). Also there is an increased risks of miscarriage. In fact, some researchers believe that a good number more of human pregnancies may begin as twins but the higher loss rate reduces the multiple pregnancy to a singleton pregnancy or in some cases a complete miscarriage. Professor C. Bocklage of East Carolina University School of Medicine believes that one in eight conceptions begin as twins and that for every twin pair born alive, ten to twelve originally multiple pregnancies result in a singleton birth (10).

Mothers have a one in 150 chance of having fraternal twins and if they have already had fraternal twins, that chance increases to one in forty. For a woman whose mother has had fraternal twins, her chance of having fraternal twins is one in ninety. This is because the rate fraternal twins is affected by genetic factors. The rate of identical twins, on the other hand, do not appear to be influenced at all by hereditary factors (1). The incidence of fraternal twins, since it is affected by genetics, seems to vary by race. People of African decent have the highest rate of fraternal twinning showing one out of seventy births as twins. Caucasians are next with one out of eighty-eight births, then Japanese with one out of 150, and finally Chinese with one out of 300. One particular tribe in Africa has a particularly high rate of twinning. The Yoruba tribe of Nigeria exhibits one out of forty five births as fraternal twins. Also, in some places, like Japan for example, there seems to be a seasonal influence on the rate of fraternal twinning. This variation differs from country to country and in some places, like the Netherlands, the seasonal variations of twin births are parallel to those of singleton births showing that in these cases, the difference is probably due to cultural and not biological factors (8).

The study of twins, or gemellology, is an important aspect of biology. By examining twins, researchers gain knowledge of genetic and environmental factors that effect the growth and development of all people. Also, the statistical information taken on twins provides information on the influence of new technology and cultural differences (such as the average age of pregnant women) on pregnancies. Most importantly, the studies conducted on twins are intriguing in themselves providing insight into a world of twins that has interested humans for ages.

Bibliography and WWW Sources

2) Bank, Stephen P. and Kahn, Micheal D., The Sibling Bond. New York: Basic Books Publishers, 1941.

3) Kitcher, Philip , The Lives to Come. New York: Simon and Shuster,1996.

4)Koch, Helen L., Twins and Twin Relations. Chicago: The University of Chicago Press,1966.

5) Newman, Horatio, Frank N. Freeman, and Karl J. Holzinger., Twins: A Study of Heredity and Environment. Chicago: The University of Chicago Press, 1965.

6) Strickberger, Monroe W., Genetics. New York: The Macmillian Company, 1968.

The Timing of Twinning

If the twinning argument is sound, then it is false that you were once a zygote. This conclusion has been used as a premise in arguments defending the morality of things like contraception, the day-after pill, IVF and stem cell research.

In an article forthcoming in the journal Zygote, medical researcher Gonzalo Herranz from University of Navarre in Spain argues that there is not enough evidence to support the current model that proposes that twinning occurs two weeks after fertilization. Most of the argument here turn on the incompleteness of the theories of exactly how twinning occurs.

Herranz goes on to call attention to an alternative model of twinning, originally presented by López-Moratalla and Cerezo. According to this model, twinning occurs at the time when the zygote first undergoes cell division. The zygote can undergo two types of cell division: it can divide into a mass consisting of two blastomeres (cells) or turn into two zygotes. Twins result only in the second case.

If this model is correct, then the twinning argument is unsound. On the new model, we might well be identical to the zygote from which we developed. If the zygote from which we came is indeed identical to us, one could question the morality of things like contraceptive pills, the day-after pill, stem cell research and IVF (which produces more embryos than are implanted). There are, of course, lots of other considerations that go into determining whether preventing the development of an embryo is ethical, but the new model would complicate at least one line of defense of these practices.

But how realistic is the second model? To answer that question, we need to look closer at what happens after fertilization. Right after a sperm fertilizes an egg cell, the zygote starts undergoing cell division while traveling to the uterus, a journey that takes 5 to 7 days in humans. At first the cells form a loose mass surrounded by a membrane inherited from the egg cell, called the zona pellucida. At the 8-cell stage, the cells are tied together with proteins that allow for an exchange of chemicals.

During implantation, which occurs the following week, the blastocyst embeds into the tissue of the uterine wall. The inner cell mass then gives rise to the embryonic disc, which consists of two types of cells, one type becomes the embryo and the umbilical cord and the other becomes the outer membranes and the placenta.

On the traditional model of twinning, the twinning process typically occurs around the time of gastrulation, though evidence suggests that it occasionally may occur at the time of the formation of the blastocyst. The later twinning occurs the higher the chance that conjoined twins will develop.

The alternative model defended by Herranz suggests that twinning occurs at the stage of the zygote. On this model, while twinning occurs almost immediately after fertilization, physical separation of the cells likely occurs only later when we can visually observe that this happens, namely around the time of gastrulation.

While it may seem plausible that the first cell division results in two zygotes that continue to develop independently but in close proximity of each other, certain observations speak strongly against the alternative model. Blastocysts with two inner cell masses have been been observed in human IVF, suggesting that twinning can occur at the time of the formation of the blastocyst or earlier. However, blastocysts with two inner cell masses are exceedingly rare, making it unlikely that twinning typically occurs at this early stage of embryonic development. At best this piece evidence suggests that twinning could occur at the time of fertilization in rare cases, though not in the majority of cases.

Another challenge for the alternative model of twinning is to explain what makes the cells after the first division of the zygote different zygotes as opposed to a mass consisting of two (or more) cells. To be zygotes it seems that they must differ in a relevant way in their internal properties or be spatially separated in away that corresponds to the later physical separation around the time of gastrulation. But there is no evidence to suggest that the cells differ relevantly in their internal properties or alternatively are relevantly spatially separated. The lack of evidence for this counts strongly against the alternative model of twinning.

There is a further problem with the argument against the current model of twinning. The argument moves from the observation that we do not really know how and why twinning takes place to the conclusion that the current model of the timing of twinning is "not based on facts." This, however, is a bit of a jump. While it's true that we don't know how and why twinning takes place, the evidence does suggest that twinning typically occurs close to a couple of weeks after fertilization.

Modelling mothers

To resolve these questions, together with colleagues Bob Black and Rick Smock, we constructed simulations and mathematical models fed with data on maternal, child and fetal survival from real populations.

This allowed us to do something otherwise impossible: control in the simulations and modelling whether women ovulated one or two eggs during their cycles. We also modelled different strategies, where we switched women from ovulating one egg to ovulating two at different ages.

We could then compare the number of surviving children for women with different patterns of ovulation.

Women who switched from single to double ovulation in their mid-20s had the most children survive in our models – more than those who always released a single egg, or always released two eggs.

This suggests natural selection favours an unconscious switch from single to double ovulation with increasing age.

Identical twins, or monozygotic twins, form when one fertilized egg splits in two and grows into two separate embryos. The twins are the same gender, share the same blood type, and share the same physical traits. Identical twins may or may not share one amniotic sac.

One of the most common questions asked is “Do identical twins have the same DNA?” They actually do have the same DNA at birth but eventually the DNA becomes more distinctive based on environmental factors. This is how each twin evolves to be a unique individual. For more on your little one’s development, check out what happens week-by-week during a twin pregnancy.

When does identical twinning happen? - Biology

Published: Sunday 05 May 2019

Written by Hannah Brown, University of Adelaide, for The Conversation

The first set of sesquizygotic, or semi-identical twins, have been identified in Australia. How is this type of twinning different from more commonly occurring identical and non-identical twins?


We all know a set of twins perhaps even a set of identical twins. In Australia, twins account for about one in 80 births.

But in research from the University of New South Wales and the Queensland University of Technology published today in the New England Journal of Medicine, we learn of a very unique set of Australian twins.

A boy and a girl from Brisbane, aged four, have been identified as only the second set of semi-identical, or sesquizygotic twins, in the world. They are the first to have been observed in utero (in the womb).

This extremely rare phenomena is the result of two sperm fertilising the same egg.
(Read more: Seeing double: why twins are so important for health and medical research)

How do twins occur?

The most common type of twins are non-identical twins, which can be the same or different sexes. Non-identical twins are also known as fraternal twins or dizygotic twins (from two zygotes, what we call the earliest embryo when the egg and sperm fuse).

This type of twinning occurs when more than one egg is released from the ovary at ovulation (normally just a single egg is selected and released), and both of the eggs are fertilised by different sperm. These twins are no more genetically similar than siblings born years apart.

The rates of non-identical twins differ between groups: it’s about eight in 1,000 in caucasian populations, 16 in 1,000 in African populations, and four in 1,000 in people of Asian decent. This suggests there is a genetic component to non-identical twins.

Identical twins (also known as monozygotic – originating from one zygote) are less common. They are the product of just one egg and one sperm, which originally form one embryo, but which break into two during the earliest stages of pregnancy. Scientists are continuing to explore how this occurs biologically, but the process remains a mystery.

The rate of identical twins is consistent across the globe at about four in 1,000 births. This suggests it’s probably just a random biological phenomena not influenced by genetics.

Prior to today, there was only one reported case of a third type of twins – semi-identical twins.

(Diagram: showing three types of twinning, copyright UNSW)

In 2006, twins in the US were identified as semi-identical twins when they were examined as infants for another medical condition. They too were the result of one egg fusing with two sperm.

But now we know the Queensland siblings are the second set of twins to fall into this mysterious and fascinating category.

What are semi-identical twins?

Scientists believe semi-identical twins are the result of one egg allowing two sperm in simultaneously (which has previously been thought to be non-viable, meaning a pregnancy would never occur).

In the case reported in today’s research, the pregnancy was identified as twins at six weeks. The ultrasound showed they shared a placenta, which is common to 70% of identical twins.

But at 14 weeks of pregnancy, tests revealed the twins were non-identical – one was a girl and the other a boy. Ordinarily non-identical twins would have their own placenta. They were an anomaly.

Thorough DNA testing has revealed they are identical on the maternal side (confirming they came from just one egg, like identical twins). But they are more like non-identical twins on the paternal side, sharing only a proportion of their father’s DNA.

Scientists understand biologically how the DNA of our mother and father mix to create an embryo (and subsequently a baby). On the day the egg and sperm meet in the fallopian tube, the DNA, packaged into chromosomes, divides equally into two, allowing the baby to inherit one copy of information from mum and one from dad.

When this doesn’t go to plan, a baby may get too many or too few chromosomes, resulting in genetic disorders like Down Syndrome (an extra copy of Chromosome 21). This may also result in a pregnancy which is not viable.

We still don’t know a lot

There is no scientific precedent for how one embryo manages to separate three sets of chromosomes, as is the case in semi-identical twins. This may remain a scientific mystery.

We have no idea how similar these twins are going to look, although the best guess is that they will be like every other set of non-identical, fraternal twins.

As they are just the second case of this type of twins ever identified, it’s hard to know if they will have a special connection beyond their extra shared DNA.

A search by researchers through huge databases of twins across the globe failed to find more semi-identical twins, suggesting this type of twinning is very rare.

Discoveries like this that teach us just how much there is still to learn about biology and health, and how fascinating the world of reproduction and pregnancy is.

Solving the Evolutionary Puzzle of Twinning

Twins have long been a source of fascination in human societies. Mythologies all over the world use twins to convey hidden messages, and literature repeatedly exploits their potential to amuse, or sometimes unnerve, an audience. But from an evolutionary point of view, they’re a puzzle. Twins have much higher mortality rates in early life than children born in single births and also pose a risk to the mother, increasing her likelihood of dying in childbirth. We seem to be a species designed to give birth to one infant at a time, like other large mammals. Why then has natural selection not weeded out the tendency for women to give birth to twins?

The answer may be that, while twinning itself carries risks, the underlying trait which leads to the most common type of twinning confers benefits. This trait is polyovulation – the tendency for women to release more than one egg from the ovaries each month. Dizygotic (or non-identical) twins result from the fertilisation of two eggs (‘zygote’ means fertilised egg), in contrast to identical (monozygotic) twinning, which happens when a single egg is fertilised but then dividing into two embryos.

Polyovulation confers benefits because it counteracts the surprisingly high likelihood of foetal loss. Not all the eggs a woman releases each month result in a successful pregnancy, even if she is actively trying to get pregnant. Some may not be fertilised, and most fertilised eggs don’t survive to full term. Exactly why isn’t clear, but this foetal loss may often be to do with chromosomal abnormalities which mean the foetus isn’t viable.

There is little data on exactly what proportion of fertilised eggs are lost, given that most seem to be lost before the woman herself is aware of the pregnancy. Identifying very early pregnancy loss requires monitoring of women’s reproductive hormones over time to pinpoint the hormonal changes which indicate pregnancy and then pregnancy loss. One of the rare pieces of evidence available, from a study in Bangladesh, suggests that very high proportions of foetal loss are common, particularly in older women: 45% foetal loss at age 18, rising to 92% by age 38.

Releasing more than one egg each month is therefore a way of ‘bet-hedging’: increasing the likelihood that at least one egg will result in a livebirth. Occasionally, more than one fertilised egg will make it through to term – and twins, or other types of multiple birth, such as triplets, result. So it’s polyovulation, the ability to release more than one egg at a time, which is favoured by natural selection, rather than twinning itself.

Another twinning puzzle: Why does the likelihood of twinning vary by maternal age?

This idea that twins are the ‘byproduct’ of an evolutionary advantage to polyovulation has been around for a while, but it doesn’t fully explain another twinning puzzle: its relationship with maternal age. Young mothers are relatively unlikely to give birth to twins the chance of twinning increases with maternal age, but then declines again in the oldest mothers.

A recent paper I’ve contributed to, led by Wade Hazel and Joseph Tomkins, has just proposed a new explanation: that polyovulation is a ‘conditional strategy’, dependent on the mother’s age. This hypothesis assumes that young women produce only one egg at a time – their risk of foetal loss is low, so they don’t ‘need’ (in an evolutionary sense) to polyovulate but at some point, a switch to polyovluation happens – older women have a much higher chance of foetal loss, so natural selection favours a switch to polyovulation to increase their likelihood of successful pregnancy.

Wade and his colleagues used mathematical simulations to test the plausibility of their idea. These models compared the reproductive success of different hypothetical strategies – a strategy of always single ovulating was compared with a strategy of always double ovulating, and with a conditional strategy of switching from single to double ovulation. The age at which switching happens was allowed to vary, to identify the best time to switch from single to double ovulation. This analysis showed that a conditional strategy, where switching to double ovulation occurred at around age 25, resulted in the highest reproductive success, in terms of leading to the most surviving children per woman by the end of her reproductive life (Figure 1).

Figure 1. Average number of offspring surviving to 15 years from simulations of the reproductive lives of 1000 women the x-axis shows the age at which ‘conditional strategists’ (those who adopt a conditional strategy of first single ovulating then double ovulating) switch from single to double ovulation. The purple box indicates women who always double ovulate, yellow those who always single ovulate, blue those who adopt a conditional strategy.

In evolutionary terms, strategies with the highest reproductive success are likely to be favoured by natural selection, so that this analysis provides support for the hypothesis that polyovulation is a conditional, age-dependent strategy.

This hypothesis is also able to explain why the frequency of twinning rises and then declines with maternal age. Younger women rarely have twins because they mostly single ovulate double ovulation – and therefore the risk of twinning – increases as women age but then at the oldest maternal ages, foetal loss is so high that, even with double ovulation, few embryos successfully make it through to term and the likelihood of twinning declines again.

What are the implications of twinning as an age-dependent, conditional strategy?

An age-dependent, conditional strategy of polyovulation could help explain why twinning rates are increasing in high income populations. Part of this increase may be due to reproductive technology, which often mimics polyovulation by implanting more than one embryo in women experiencing fertility treatment. But in high income populations women often delay first births until into their late 20s and 30s. If polyovulation is more likely in women in these age groups, then rates of twinning will be higher in populations where women have children relatively late, even without assisted reproduction.

These combined characteristics of polyovulation plus high rates of foetal loss also imply that many of us once had a twin. The ‘vanishing twin’ phenomenon, where a twin pregnancy detected early on subsequently becomes a singleton pregnancy, is well known in obstetrics this new model suggests the frequency of this phenomenon is likely to be high, particularly among older mothers.

Twinning is a relatively rare event, varying from around 0.6 to 4% of all births, and this new paper suggests that its rarity is because it is an ‘accident’ arising from a strategy of polyovulation to counteract very high rates of foetal loss. Many questions still remain about this phenomenon: such as why foetal loss is so high, and why twinning rates vary around the world – because of differences in the likelihood of foetal loss, perhaps? But regardless of the explanation for the existence of twins, mythology, literature and the human experience is much richer because of this fascinating phenomenon.

Hazel, N. W., Black, R., Smock, R.C., Sear, R., Tomkins, J.L. (2020) An age-dependent ovulatory strategy explains the evolution of dizygotic twinning in humans. Nature Ecology & Evolution. DOI: 10.1038/s41559-020-1173-y

Published On: May 11, 2020

Rebecca Sear

Rebecca Sear is a Reader at the London School of Hygiene and Tropical Medicine (LSHTM), teaching demography and researching human reproductive behaviour from an evolutionary perspective. She is trained in zoology, biological anthropology, and statistics, and subsequently worked first in a social science institution (London School of Economics) and then in an institution of global and public health (LSHTM). Having been exposed to a variety of disciplines, she is particularly interested in how the natural, social and medical sciences can be integrated as we try to understand our own species, and aims to conduct research somewhere inbetween these disciplines. She is particularly interested in taking a comparative perspective to understanding human reproductive behaviour, and exploring why such behaviour varies between, as well as within, populations.

Characteristics of Mirror Twins

Mirror twins have certain physical characteristics in common—these similarities are just reflected on opposite sides of their bodies.

Physical Traits

Potential characteristics that could look alike in mirror twins include: birthmarks, moles, freckles, dimples, eye or eyebrow shapes, nostril or ear shapes, cowlicks, hair whorls, or teeth.

For example, a similar birthmark may manifest on the left side of one twin and on the right side of the other. Facial features such as dimples may be on opposite sides of the face. Cowlicks may run clockwise on one twin and counterclockwise on the other.

Gestures or Movements

Gestures or movements can also be manifestations of mirror image twinning. For example, one twin may be right-handed and the other left-handed (although many twins, regardless of zygosity, share this characteristic). Additionally, one twin may prefer to sleep on their left side while the other prefers the right.

Medical Conditions

In some extreme cases (which are exceptionally rare), mirror twins display situs inversus. In this condition, the internal organs (such as the heart, liver, lungs, or stomach) are situated on the opposite side of the normal anatomical position. An X-ray, CT scan, MRI, or ultrasound assessment can be used to identify the position of internal organs when situs inversus occurs.

There are also less serious medical conditions that have a higher chance of occurring with mirror twins. These conditions could be benign or require surgery to make them more manageable:

  • Accessory (extra) toes
  • Bone cysts
  • Cleft lip and cleft palate
  • Esotropia (eye misalignment)
  • Supernumerary (extra) teeth
  • Trigger thumb, or having a thumb locked in a bent position

Facial asymmetry and abnormal dental development may also be observed in mirror image twins.

A strategy for prolonging fertility

The reason a switch is beneficial is fetal survival – the chance that a fertilised egg will result in a liveborn child – decreases rapidly as women age

So switching to releasing two eggs increases the chance at least one will result in a successful birth.

But what about twinning? Is it a side effect of selection favouring fertility in older women? To answer this question, we ran the simulations again, except now when women double ovulated the simulation removed one offspring before birth.

In these simulations, women who double ovulated throughout their lives, but never gave birth to twins, had more children survive than those who did have twins and switched from single to double ovulating.

This suggests the ideal strategy would be to always double ovulate but never produce twins, so fraternal twins are an accidental side effect of a beneficial strategy of double ovulating.

Joseph L Tomkins, Associate Professor in Evolutionary Biology, University of Western Australia Rebecca Sear, Head of the Department of Population Health, London School of Hygiene & Tropical Medicine, and Wade Hazel, Professor of Biology, DePauw University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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