Fraternal twins run in families, and the evidence for that is strong. Identical twins, by contrast, have long been considered a biological fluke with no clear genetic basis, though newer research is complicating that picture. The distinction matters because “twins” is not one phenomenon. Fraternal twins come from two separate eggs fertilized by two separate sperm, while identical twins come from a single fertilized egg that splits. These are fundamentally different biological events, and their relationship to heredity is just as different.
Why Fraternal Twinning Runs in Families
The key to fraternal (dizygotic) twinning is hyperovulation, the release of two or more eggs in a single menstrual cycle. If a woman’s body tends to release multiple eggs, she has a higher chance of conceiving fraternal twins. That tendency is partly genetic. Researchers have found that mothers of hereditary dizygotic twins have significantly elevated levels of follicle-stimulating hormone (FSH), the hormone that triggers egg development. In one study, nearly half of premenopausal mothers of twins had abnormally high FSH levels, compared to roughly one in fourteen among control subjects.1PubMed. Increased levels and pulsatility of follicle-stimulating hormone in mothers of hereditary dizygotic twins The elevated FSH was driven by faster hormone pulses from the brain, not by differences in the ovaries themselves. In other words, these women’s bodies were simply sending a stronger “go” signal to the ovaries.
That hormonal tendency is inherited. Large genome-wide studies have pinpointed specific stretches of DNA associated with dizygotic twinning. The first major study identified variants near the gene FSHB, which encodes part of the FSH molecule itself, and another in the gene SMAD3, which is involved in how ovarian cells respond to growth signals.2American Journal of Human Genetics. Genome-wide association analyses identify 18q11.2 and 15q21.3 as susceptibility loci for dizygotic twinning A more recent and larger meta-analysis confirmed those two loci and discovered two more, one near a gene called GNRH1 on chromosome 8 and another near ZFPM1 on chromosome 16.3Oxford Academic. Genome-wide association study meta-analysis of dizygotic twinning illuminates genetic regulation of female fecundity GNRH1 codes for the hormone that tells the pituitary gland to release FSH in the first place, so the genetic story converges neatly on the same biological pathway: variants that ramp up FSH signaling lead to more eggs released per cycle, which leads to more fraternal twins.
Each of these gene variants individually has a modest effect. No single variant guarantees someone will have twins. But when several variants combine in one person, the cumulative effect on ovulation can be substantial. That is why twinning sometimes seems to “skip” generations or appear unpredictably in families: the right combination of variants has to come together.
Does the Father’s Side Matter?
This is one of the most common questions people have, and the answer is nuanced. Because fraternal twinning depends on a woman releasing two eggs, the genetics that matter most are hers. A man who is himself a fraternal twin, or whose mother was, carries no biological mechanism to make his partner release extra eggs. His genes play no direct role in whether she ovulates once or twice.
However, a father can pass twinning-associated gene variants to his daughters. If he carries variants near FSHB or SMAD3, for example, his daughter could inherit them and have a higher chance of hyperovulating when she reaches reproductive age. So the “twin gene” can travel silently through a man and express itself in the next generation of women. This is the biological basis of the popular idea that twinning skips a generation. It doesn’t literally skip, but it can appear to, because the genetic contribution from a father’s side stays invisible until it reaches a daughter.
Maternal Age, Body Size, and Other Non-Genetic Factors
Genetics is only part of the fraternal twinning story. A woman’s age has a well-documented effect: dizygotic twinning rates climb steadily through a woman’s thirties and into her early forties. This happens because FSH levels naturally rise as women age, a compensatory response to declining ovarian reserve that sometimes triggers the release of multiple eggs.4Human Reproduction. The paradox of declining fertility but increasing twinning rates with advancing maternal age Norwegian data confirmed this pattern, showing that the odds of naturally conceived twins increased progressively with maternal age, reaching about 36% higher odds in the 40–44 age group compared to earlier cohorts.5PubMed. Increasing twinning rates in Norway, 1967-2004: the influence of maternal age and assisted reproductive technology (ART)
Eventually twinning rates decline again in the mid-forties as overall fertility drops off sharply. One evolutionary model frames this as an adaptive strategy: as egg quality declines with age, releasing multiple eggs per cycle increases the chance that at least one will result in a viable pregnancy.6Nature Ecology & Evolution. An age-dependent ovulatory strategy explains the evolution of dizygotic twinning in humans
Body size also plays a role. A large U.S. study found that women with a BMI of 30 or higher had significantly elevated odds of dizygotic twinning, even after adjusting for age, race, parity, and height. Taller women also had increased odds, though the effect was smaller than that of higher BMI. Importantly, neither BMI nor height was linked to identical twinning, reinforcing that the two types of twinning operate through different mechanisms.7PubMed. Relationship of maternal body mass index and height to twinning The likely explanation is that higher body fat is associated with higher levels of insulin-like growth factor, which can stimulate the ovaries alongside FSH.
Global Variation in Twinning Rates
If twinning were purely genetic and those genes were evenly distributed, you would expect roughly similar twin rates worldwide. That is not what we see. The variation across populations is enormous and points to both genetic and environmental influences shaping twinning patterns.
Central and West Africa have the highest natural twinning rates on the planet. A large survey of the developing world found that a zone of high twinning, above 18 per 1,000 births, stretches from Guinea along the Atlantic coast through to Congo, Tanzania, and Mozambique. Benin recorded the highest national rate at nearly 28 per 1,000.8PubMed Central. Twinning across the Developing World At the other extreme, most of South and Southeast Asia, including China, India, Vietnam, and Indonesia, had rates below 9 per 1,000. Latin America largely mirrored Asia’s low rates.
This variation is driven almost entirely by differences in dizygotic twinning. Identical twinning occurs at a relatively stable rate of about 3–4 per 1,000 births worldwide, regardless of population. The massive differences between Africa and Asia reflect the frequency of hyperovulation-promoting gene variants in different populations, shaped by tens of thousands of years of evolutionary history. Diet, average body composition, and other environmental factors also contribute, but genetics appears to be the dominant driver of the population-level differences.
Are Identical Twins Hereditary?
For decades, the standard answer has been no. Identical (monozygotic) twinning was considered a random event, a spontaneous splitting of a very early embryo with no known genetic trigger. The evidence for that view was straightforward: identical twinning rates do not vary much between populations, they do not obviously cluster in families the way fraternal twinning does, and no “splitting gene” has been identified.
But there are cracks in that simple story. A study in Nature found evidence that a propensity for identical twinning can be inherited through the maternal line and even suggested that the mechanisms behind identical and fraternal twinning might be related.9PubMed. Familial incidence of twinning More recently, researchers discovered that identical twins carry a distinctive DNA methylation signature, a pattern of chemical marks on their genome that differs from non-twins. These marks are enriched near telomeres and centromeres, in regions associated with cell adhesion and WNT signaling (pathways involved in how cells stick together and decide their fate), and at sites linked to early embryonic programming.10Nature Communications. Identical twins carry a persistent epigenetic signature of early genome programming
The implication is provocative. If identical twins share an epigenetic signature that persists across tissues and throughout life, it may be that certain embryos are predisposed to split because of how their genomes are programmed around the time of implantation. A large fraction of these distinctive methylation sites overlap with so-called metastable epialleles, loci where the methylation state is established very early in development and remains stable across the body.11Human Reproduction. New insights into the (epi)genetics of twinning This coincides with the window when most identical twinning events are thought to occur. So while no one has found a classic “identical twin gene,” the splitting event may not be purely random either. It could be influenced by how the early embryo’s genome is chemically tagged, and that tagging process itself could have heritable components.
Sesquizygotic Twins and Other Rare Types
Most discussions of twins divide them neatly into fraternal and identical. But biology doesn’t always cooperate with neat categories. In 2019, researchers documented a type of twinning that falls between the two: sesquizygotic twins. These twins are suspected to result from a single egg being fertilized by two separate sperm, followed by a division of the resulting cell mass into two embryos.12PubMed. The New Sesquizygotic Twins and More: Exotic Twin Types Genotyping of the twins confirmed chimerism arising at the juncture of cell division, a process termed heterogonesis.13PubMed. Molecular Support for Heterogonesis Resulting in Sesquizygotic Twinning
Sesquizygotic twins share all of their mother’s DNA but only a portion of their father’s, making them genetically somewhere between identical and fraternal. The condition is extremely rare, and it is unclear whether it has any hereditary component. Laboratory studies in cattle have shown that multipolar cell divisions during the earliest stages after fertilization can produce cells carrying only one parent’s genome, demonstrating that the underlying cellular machinery for these errors exists in mammals generally.14PubMed Central. Parental genomes segregate into distinct blastomeres during multipolar zygotic divisions leading to mixoploid and chimeric blastocysts But for now, sesquizygotic twinning is best understood as a biological curiosity rather than a heritable trait.
The Vanishing Twin Problem
One underappreciated factor in understanding twin heredity is that many twin pregnancies go undetected. Vanishing twin syndrome, where one twin is lost in the first trimester, occurs in an estimated 15–35% of twin pregnancies.15PubMed. The vanishing twin: Diagnosis and implications Before routine early ultrasound, most of these were never known to have been twin pregnancies at all. An older ultrasound study of 1,000 first-trimester pregnancies found a minimum twinning incidence of about 3.3%, with over a fifth of those cases showing the vanishing twin phenomenon. The study concluded that the true incidence of multiple gestation is higher than previously believed, potentially reaching above 5%.16PubMed. The “vanishing twin”: ultrasonographic assessment of fetal disappearance in the first trimester
This matters for heredity research because family histories of twinning are almost certainly undercounted. A woman who hyperovulates and conceives fraternal twins frequently may deliver singletons if one embryo is lost early. Her genetic predisposition to twinning is real, but it would never show up in birth records. This means the actual heritability of twinning could be higher than estimates based on delivered twins alone would suggest.
Twinning Mothers as a Group
An interesting finding from evolutionary biology adds another layer. Using a large historical database from Utah, researchers compared over 4,600 mothers who bore twins with more than 54,000 who had only singletons. The twin-bearing mothers had shorter intervals between births, later ages at last birth, higher lifetime fertility, and lower postmenopausal mortality.17PubMed Central. Twinning in humans: maternal heterogeneity in reproduction and survival In other words, mothers of twins were not depleted by the extra demands of a multiple pregnancy. They appeared to be more physically robust across the board.
This finding supports the idea that twinning is a marker for overall maternal vigor rather than just a quirk of ovulation. Women who carry the genetic variants for hyperovulation may also carry variants for generally stronger reproductive health. If that is the case, the “twin gene” is less a narrow switch for releasing two eggs and more a cluster of traits that together signal a body well-equipped for reproduction. That would help explain why the trait persists in human populations despite the real risks that twin pregnancies carry.
Fertility Treatments and Modern Twinning Rates
Any discussion of twin heredity in 2024 has to account for the elephant in the room: assisted reproductive technology. Fertility treatments, especially ovarian stimulation drugs and in vitro fertilization with multiple embryo transfers, have dramatically increased twinning rates in wealthy countries over the past few decades. This creates a challenge for genetic research, because many twin pregnancies in modern datasets are the result of medical intervention rather than natural hyperovulation.
The major genetic studies accounted for this by carefully screening out mothers who used fertility treatments. The genome-wide association study that identified the FSHB and SMAD3 loci, for instance, excluded all cases involving assisted reproduction.2American Journal of Human Genetics. Genome-wide association analyses identify 18q11.2 and 15q21.3 as susceptibility loci for dizygotic twinning So the genetic findings reflect natural twinning biology, not drug-induced ovulation. Still, if you meet someone who is a fraternal twin born after 1980 in a developed country, the probability that fertility treatment played a role is considerably higher than it would have been for someone born in 1960. That does not mean their family has no genetic tendency toward twins, but it does mean the family pattern is harder to interpret.
Interestingly, there is a genetic overlap between natural twinning and the response to fertility drugs. The same FSHB variants that predispose to spontaneous dizygotic twinning also affect how a woman’s ovaries respond to exogenous gonadotropins. Women carrying certain variants may over-respond to standard fertility drug doses, which is one reason clinicians try to personalize dosing during ovarian stimulation.
What Twin Studies Have Taught Us About Everything Else
Twins are not only the subjects of genetic research; they are one of the most powerful tools in it. The classic twin study design, comparing how similar identical twins are versus fraternal twins on any given trait, has been used since the early twentieth century to estimate how much of the variation in a trait comes from genes versus environment.18PubMed Central. Twin Studies: A Unique Epidemiological Tool Height, weight, personality, disease risk, intelligence, and hundreds of other traits have been studied this way. Despite the simplicity of the underlying assumptions, the heritability estimates from classical twin studies have generally held up well when checked against modern molecular genetics.
But one of the most surprising findings from twin research has been how identical twins can diverge over time. A landmark study found that while young identical twins are epigenetically almost indistinguishable, older identical twins show striking differences in their DNA methylation and histone modification patterns.19PubMed Central. Epigenetic differences arise during the lifetime of monozygotic twins These accumulated epigenetic changes affect which genes are active, helping to explain why identical twins can develop different diseases, have different body compositions, or age at different rates despite sharing the same DNA sequence.
The sources of this divergence go beyond just lifestyle differences after birth. Differences in the intrauterine environment, genetic mosaicism arising from mutations after the embryo splits, and purely random (stochastic) variation all contribute.20PubMed Central. Genetic, environmental and stochastic factors in monozygotic twin discordance with a focus on epigenetic differences Two identical twins can receive unequal shares of the placental blood supply, leading to measurable differences from the moment of birth. So even the most “identical” twins are never truly identical in every biological sense. Their shared genome is a starting point, not a fixed destiny.
Twinning in Other Mammals
Humans are not the only species where twinning has a genetic basis, and studies in livestock have actually moved faster than human research in identifying the specific genes involved. Sheep, cattle, and goats have been selectively bred for multiple births for centuries, and researchers have identified mutations in genes like BMP15, GDF9, and BMPR1B that dramatically increase ovulation rate. These genes are part of an intra-ovarian signaling pathway, and their discovery revealed that signals from the egg cell itself are a primary driver of follicle development, more so than circulating hormones alone.21Reproduction, Fertility and Development. Genetic regulation of ovulation rate and multiple births
Human versions of these genes exist, but they have not yet been convincingly linked to natural twinning in people. The gene variants that control sheep twinning tend to have large effects because they were concentrated through selective breeding, a process that creates much stronger genetic signals than natural selection typically produces. In humans, the genetic architecture of twinning involves many variants with small individual effects, which is why it took large-scale genome studies to find them. Still, the sheep research has been invaluable for understanding the basic biology of ovulation rate, and it is entirely possible that rare mutations in human BMP15 or GDF9 contribute to twinning in some families, just in ways that are hard to detect in population-wide studies.