Twins do not actually skip a generation. The idea is one of the most persistent myths in family folklore, but it confuses a real genetic pattern with something it is not. The grain of truth behind the saying involves fraternal twins specifically, and the way genes for double ovulation can travel silently through a father who has no ovaries to express them. The full picture involves at least six identified genes, a handful of non-genetic factors, and one type of twin that has almost no heritable pattern at all.
How the Myth Got Started
The “skipping a generation” idea follows a specific family story that many people recognize. A woman has fraternal twins. Her son inherits whatever genetic tendency she had toward releasing two eggs at once, but because he does not ovulate, the trait sits invisibly in his DNA. He passes the relevant gene variants to his daughter, who does ovulate, and she has twins. From the family’s perspective, twins appeared in grandma’s generation, disappeared in dad’s, and resurfaced in the granddaughter. It looks like a skip.
But the gene did not skip anything. It was present in every generation. The father carried it the whole time. He just could not express it in a way that produces twins, because the trait controls a female reproductive process. If that same man had a sister instead, she could have had twins in the very next generation with no “skip” at all. The pattern people notice is really about which sex inherits the gene and whether that person has ovaries, not about some mysterious generational leap.
There is another, simpler reason the myth persists: small families. When a woman carries hyperovulation genes but happens to have only one or two pregnancies, she may never produce twins by sheer probability. Her daughter, with more pregnancies or just different luck, does. The family reads this as skipping when it was actually chance all along.
Fraternal Twins Have a Genetic Basis
Fraternal (dizygotic) twins happen when a woman releases two eggs in a single cycle and both get fertilized. This tendency, called hyperovulation, runs in families and has a clear genetic component. A large genome-wide study identified two key gene variants involved: one near a gene called FSHB, which helps regulate follicle-stimulating hormone, and another within SMAD3, whose protein plays a role in how the ovaries respond to that hormone.1American Journal of Human Genetics. Meta-analysis identifies common variants associated with dizygotic twinning A follow-up meta-analysis found four additional gene regions tied to fraternal twinning, including variants in GNRH1 and FSHR, both of which are involved in the hormonal chain that triggers ovulation.2Human Reproduction. Genome-wide association study meta-analysis of dizygotic twinning illuminates genetic regulation of female fecundity
What these genes have in common is that they all touch the hormonal pathway controlling how many eggs mature and release each cycle. Women who carry certain combinations of these variants are more likely to release two eggs at once. This is why fraternal twinning clusters in families and why it is the only type of twinning with a well-understood hereditary mechanism.
One detail that complicates the simple “maternal line only” story: there is evidence that fathers contribute to fraternal twinning too, not just by silently carrying hyperovulation genes for their daughters, but potentially through sperm-related factors. Research has found that producing fraternal twins requires not only a woman who releases multiple eggs but also sperm capable of fertilizing more than one egg in a given cycle.3PubMed. Mechanisms of twinning: X. The male factor One hypothesis suggests that fathers in twinning-prone families carry gene variants that increase the chance of two sperm successfully fertilizing two eggs.4Twin Research and Human Genetics. Paternal Familial Twinning: Hypothesis and Genetic/Medical Implications This is still an area of active research, but it means the old rule of “twinning only comes through the mother” is an oversimplification.
Identical Twins Are a Different Story
Here is where many people get confused. Identical (monozygotic) twins form when a single fertilized egg splits into two embryos. This is a fundamentally different event from fraternal twinning, and for a long time scientists believed it was essentially random, with no genetic influence at all. The reality is more nuanced but still quite different from the fraternal twin story.
One older study found evidence that a tendency toward identical twinning can be inherited through the maternal line, and that the mechanisms behind identical and fraternal twinning might even be related.5PubMed. Familial incidence of twinning More recently, researchers discovered that identical twins carry a persistent epigenetic signature, a distinct pattern of chemical modifications on their DNA that appears to be set very early in development.6Nature Communications. Identical twins carry a persistent epigenetic signature of early genome programming This suggests the splitting event is not purely random but may be influenced by molecular factors present around the time of conception.
Still, there is no “identical twin gene” that families pass down in any predictable way. Identical twinning occurs at roughly the same rate across populations worldwide, about one in every 250 births, regardless of family history or ethnicity. So when someone says “twins run in my family,” what they almost always mean, genetically speaking, is fraternal twins. And the “skipping a generation” myth applies only to that type.
Age, Body Size, and Parity
Genetics is only part of the fraternal twinning equation. Several non-genetic factors substantially change a woman’s odds, and these help explain why twins can seem to appear and disappear unpredictably within a family tree.
Maternal age is one of the strongest predictors. As women get older, hormonal changes make it more likely that two eggs will be released in a single cycle. Follicle-stimulating hormone levels rise with age, and this increases the chance of hyperovulation even in women who do not carry the gene variants associated with it. This age effect is significant enough that delayed childbearing has been identified as one of the two main drivers behind the dramatic increase in twinning rates in developed countries over the past four decades.7Population and Development Review. Twinning Rates in Developed Countries: Trends and Explanations
Body size matters too. After adjusting for age and other factors, women with a BMI of 30 or higher had significantly elevated odds of fraternal twinning, and taller women also showed an increased risk, though the height effect was smaller than the weight effect.8Obstetrics and Gynecology. Relationship of Maternal Body Mass Index and Height to Twinning A review confirmed that taller mothers and those with a high BMI face greater odds of fraternal twins.9Human Reproduction Update. Dizygotic twinning The mechanism is thought to involve insulin-like growth factor, which is higher in taller and heavier women and may stimulate the ovaries to release multiple eggs.
Parity also plays a role. Women who have already had several pregnancies are more likely to have fraternal twins than women in their first pregnancy. When you combine all of these factors, a woman who is older, taller, heavier, and has had previous children is substantially more likely to have twins than a younger, smaller first-time mother, regardless of family history. This means the “skipping” pattern in many families may reflect changing demographics across generations as much as anything genetic.
Ethnicity and Geography
Fraternal twinning rates vary enormously across populations, and this variation is almost entirely about fraternal twins. The Yoruba people of Nigeria have a twinning rate more than four times that of European populations, driven by much higher proportions of fraternal twins.10PubMed. The Yoruba contribution to our understanding of the twinning process Globally, the highest fraternal twinning rates are found in sub-Saharan African populations, while the lowest are found in East Asian populations, with European populations in between.11PubMed Central. Frequency of twinning in southwest Nigeria
These population-level differences reflect the varying frequency of hyperovulation gene variants across ethnic groups. Diet may also contribute: yams are a staple food among the Yoruba and contain phytoestrogens that some researchers have speculated could stimulate ovulation, though this has never been conclusively demonstrated. What is clear is that the baseline probability of fraternal twins differs dramatically depending on ancestry, which means the “skipping” pattern would look very different in a Yoruba family than in a Japanese one simply because the underlying gene frequency is different.
Fertility Treatments Changed the Math
Any discussion of twinning patterns in modern families has to account for the massive impact of assisted reproduction. By 2011, an estimated 36% of twin births and 77% of triplet and higher-order births in the United States resulted from fertility treatments.12PubMed. Fertility treatments and multiple births in the United States IVF has become one of the most common treatments for infertility, and its widespread use has substantially increased the incidence of twin pregnancies.13PubMed Central. The clinical characteristics differences between IVF twins and naturally conceived twins with preterm infants
This matters for the “skipping” myth because many families who have twins through fertility treatments assume the twins are genetic, reinforcing the idea of inherited twinning even when the cause was medical. Conversely, families that might have had twins naturally never do because they completed their families with fewer pregnancies, while other families that would not naturally twin end up with twins through IVF. The effect of medically assisted reproduction on twinning rates has been estimated to be about three times as important as the effect of delayed childbearing.7Population and Development Review. Twinning Rates in Developed Countries: Trends and Explanations All of this scrambles the family patterns people try to read as evidence for or against generational skipping.
The Folic Acid Question
One recurring claim is that folic acid supplements increase the chance of having twins. This gained traction after some studies found a statistical association between preconception folic acid use and twin pregnancies. However, the best evidence suggests the association is largely an artifact of who takes folic acid. Women undergoing IVF are more likely to take folic acid and more likely to have twins, creating a misleading link between the two. One study found an apparent increased risk with folic acid use, but after excluding IVF pregnancies and accounting for underreporting, the association disappeared entirely.14PubMed. Folate supplementation and twin pregnancies
A systematic review reached a similar conclusion: after adjusting for major confounders, the best estimate for any increased risk of fraternal twinning with preconception folic acid was modest and not statistically significant.15Medical Journal of Australia. Why Do Twins Skip a Generation? The Myth and the Science One study from the Netherlands did find a stronger association, with roughly doubled odds of fraternal twinning among folic acid users in certain years, though this study did not fully account for IVF confounding.16Early Human Development. Use of folic acid supplementation and risk for dizygotic twinning The overall picture: folic acid probably does not meaningfully increase your chance of twins, and the recommendation to take it before and during early pregnancy for neural tube defect prevention should not be avoided out of twin-related fear.
When Twins Vanish Before Birth
An underappreciated wrinkle in the twinning story is that more twin pregnancies begin than end. Vanishing twin syndrome describes cases where an early ultrasound at six or seven weeks shows two embryos but a later scan near the end of the first trimester shows only one surviving fetus.17Best Practice & Research Clinical Obstetrics & Gynaecology. The vanishing twin: Diagnosis and implications The tissue from the lost twin is usually reabsorbed by the mother’s body or compressed into the placenta. Before routine early ultrasound, most families never knew a twin had been conceived at all.
This has a subtle connection to the “skipping” myth. In some families, twins may have been conceived across multiple generations but never born as twins because one embryo was lost early. The apparent absence of twins in a generation may reflect pregnancy loss rather than the absence of hyperovulation genes. With modern ultrasound technology, vanishing twin syndrome is detected more frequently, which means we now see twin conceptions that would have gone completely unnoticed in earlier generations.
Sesquizygotic Twins and the Space Between Identical and Fraternal
For most of medical history, twins were understood as either identical or fraternal. Then in 2019, a research team reported only the second confirmed case of an intermediate type. These sesquizygotic twins share all of their mother’s DNA, like identical twins, but are chimeric for their father’s DNA, sharing about 78% of it. The twins resulted from a single egg apparently being fertilized by two sperm, followed by a three-way cell division that produced two embryos with mixed paternal genetics.18PubMed. Molecular Support for Heterogonesis Resulting in Sesquizygotic Twinning The researchers screened nearly a thousand fraternal twin pairs and found no additional cases, confirming sesquizygotic twinning is exceptionally rare.19PubMed. The New Sesquizygotic Twins and More: Exotic Twin Types/Twin Research Reviews
Sesquizygotic twins are interesting in this context because they further undermine any neat narrative about twin inheritance. They do not fit the fraternal model of two eggs and two sperm, and they do not fit the identical model of one egg splitting. Nobody has suggested sesquizygotic twinning runs in families. It appears to be a genuinely anomalous event, a reminder that human reproduction involves enough biological complexity that occasionally something entirely unexpected happens.
Why Identical Twins Diverge Over Time
Although identical twins start life with virtually the same DNA sequence, they do not stay biologically identical. Research on large groups of identical twin pairs found that young twins are nearly indistinguishable at the epigenetic level, but older identical twins show striking differences in how their genes are chemically tagged and expressed.20PubMed Central. Epigenetic differences arise during the lifetime of monozygotic twins These epigenetic changes accumulate through different environmental exposures, diets, habits, and random molecular events over a lifetime.
This divergence has real medical consequences. Identical twins can end up discordant for diseases like diabetes, autoimmune conditions, or psychiatric disorders despite sharing the same genetic code. The epigenetic differences between them help explain why, and studying those differences has become a powerful tool for teasing apart the contributions of genes and environment to disease.21PubMed Central. Epigenetics of discordant monozygotic twins: implications for disease For identical twins themselves, the practical takeaway is that sharing DNA does not mean sharing destiny. Your twin may carry the same risk variants you do, but whether those variants get expressed depends on the life each of you lives.
Complications Unique to Shared-Placenta Twins
About two-thirds of identical twins are monochorionic, meaning they share a single placenta. This arrangement creates risks that fraternal twins and the remaining third of identical twins do not face, because sharing a placenta means sharing a blood supply. Twin-to-twin transfusion syndrome occurs in roughly 10 to 15% of monochorionic twins when blood flows unevenly between the two through vascular connections in the shared placenta.22PubMed Central. Twin to twin transfusion syndrome One twin receives too much blood while the other receives too little, and without treatment the condition carries very high mortality.
The blood vessel connections responsible exist in all monochorionic placentas, not just those that develop the syndrome. What distinguishes the roughly 10% that progress to transfusion syndrome appears to involve the specific architecture of these connections, particularly the balance between arterial-to-venous channels running in each direction.23American Journal of Obstetrics and Gynecology. Angioarchitecture of monochorionic placentas in relation to the twin-twin transfusion syndrome When those channels are imbalanced, blood drains from one twin to the other without adequate return flow.24Placenta. Placental markers of twin-to-twin transfusion syndrome in diamniotic–monochorionic twins: A morphometric analysis of deep artery-to-vein anastomoses Modern diagnosis relies on sequential ultrasound examinations showing fluid imbalances between the two amniotic sacs, and treatment options have improved substantially over the past two decades. For parents of monochorionic twins, early and frequent monitoring is standard practice specifically because of this risk.