The mother is the primary carrier of genes that influence fraternal twinning. Fraternal twins happen when a woman releases two eggs in one cycle, and the tendency to do that is driven by her own hormonal and genetic makeup. A father cannot cause a woman to release two eggs, so his direct genetic contribution to twinning is minimal. The story is more layered than that simple answer, though, because identical twins follow different rules entirely, and even the father’s side of the family tree isn’t completely irrelevant.
Why the Mother’s Genetics Drive Fraternal Twinning
Fraternal (dizygotic) twins form when two separate eggs are fertilized by two separate sperm in the same cycle. The biological bottleneck is ovulation: a woman typically releases one egg per cycle, and releasing two is the unusual event that makes fraternal twins possible. This process, sometimes called hyperovulation, runs in families on the mother’s side. Women who are themselves fraternal twins, or whose mothers or sisters had fraternal twins, are more likely to release multiple eggs.
The hormonal mechanism behind this has been studied since the late 1990s. Researchers found that premenopausal mothers of fraternal twins had significantly higher levels of follicle-stimulating hormone (FSH) and more frequent FSH pulses compared to control women. Seven out of sixteen mothers of twins in one study had abnormally high FSH levels above 10 IU/L, compared with just one out of fourteen controls.1PubMed. Increased levels and pulsatility of follicle-stimulating hormone in mothers of hereditary dizygotic twins Additional research confirmed that mothers of fraternal twins showed elevated inhibin and LH levels throughout the follicular phase, pointing to greater follicular activity overall.2Fertility and Sterility. Elevation of follicular phase inhibin and luteinizing hormone levels in mothers of dizygotic twins suggests nonovarian control of human multiple ovulation
FSH is the hormone that stimulates egg follicles to grow and mature in the ovaries. Women with naturally higher FSH activity are more likely to push two follicles past the finish line in a single cycle, resulting in two eggs available for fertilization. The fact that this trait clusters in families is what led scientists to hunt for the specific genes involved.
The Genes That Have Been Found
A large genome-wide association study comparing nearly 2,000 mothers of spontaneous fraternal twins with about 13,000 controls identified two genetic variants with strong, replicated links to fraternal twinning. One variant, located near the FSHB gene on chromosome 11, was associated with a roughly 18% increase in the chance of delivering twins for each copy of the risk allele a woman carried. A second variant, within the SMAD3 gene on chromosome 15, added about a 9% increase per copy.3PubMed Central. Identification of Common Genetic Variants Influencing Spontaneous Dizygotic Twinning and Female Fertility A third locus on chromosome 1 also reached statistical significance in the initial analysis.
The FSHB gene codes for the beta subunit of FSH itself, so a variant that cranks up FSH production or activity has a straightforward path to causing hyperovulation. The SMAD3 gene is involved in how the ovaries respond to FSH signaling. Together, these two variants explain a piece of the puzzle from both ends: how much FSH is produced and how sensitive the ovaries are to it. The FSHB variant also tracked with earlier onset of menstruation, higher lifetime number of children, and earlier menopause, all consistent with a profile of heightened reproductive drive.3PubMed Central. Identification of Common Genetic Variants Influencing Spontaneous Dizygotic Twinning and Female Fertility
These variants are common in the population, which is part of why fraternal twinning doesn’t follow a clean dominant/recessive pattern the way some textbook traits do. Many women carry one or both variants and never have twins. The genes raise the probability rather than guaranteeing the outcome, and plenty of other genetic and non-genetic factors play into whether hyperovulation actually leads to a twin pregnancy in any given cycle.
Does the Father Contribute Anything?
The conventional wisdom is that fathers play no role, because men don’t ovulate. But a few lines of evidence suggest the picture isn’t completely one-sided. A study examining families of men who fathered twins found that the tendency to father multiples appeared to be inherited along the male line in many cases. The researchers proposed that producing twins requires not only a woman capable of double ovulation but also a man whose sperm can successfully fertilize more than one egg in a given cycle.4PubMed. Mechanisms of twinning: X. The male factor
This doesn’t mean fathers carry “twin genes” in the same way mothers do. The idea is subtler: some men may produce sperm that are more capable of navigating the reproductive tract and reaching eggs efficiently, so if their partner happens to ovulate twice, the odds of both eggs being fertilized go up. The evidence for this is much weaker and more contested than the evidence for maternal genetics. Most reproductive geneticists still consider the mother’s genotype the dominant factor, and the paternal contribution, if real, a secondary amplifier rather than a trigger.
There’s also an indirect pathway. A father can pass hyperovulation-associated gene variants to his daughters, who may then be more likely to have fraternal twins themselves. So if twins run “on the father’s side” of your family, those genes may have traveled silently through one generation of men before expressing themselves in a female relative. The genes don’t skip a generation in any magical sense; they just can’t do anything in the male body and only become visible when inherited by a daughter.
Identical Twins Follow Different Rules
Everything above applies to fraternal twins. Identical (monozygotic) twins form when a single fertilized egg splits into two embryos, and the causes of that split are mostly unknown. The rate of identical twinning is remarkably stable across populations and time periods, hovering around 3 to 4 per 1,000 births worldwide, which by itself suggests the event is largely random rather than driven by inherited genetic variation.5PubMed Central. Twinning across the Developing World
The leading explanation is that identical twinning is a stochastic event during early embryo development, one that doesn’t typically cluster in families. However, this “random accident” view has been challenged. A review of the literature argued that familial monozygotic twinning is more common than previously recognized.6PubMed. Non-identical monozygotic twins, intermediate twin types, zygosity testing, and the non-random nature of monozygotic twinning: a review And an analysis of about 950 twin pairs found evidence that the tendency toward identical twinning can be inherited through the maternal line, and that the mechanisms behind identical and fraternal twinning might even be related.7Nature. Familial incidence of twinning
More recently, researchers discovered that identical twins carry a distinctive DNA methylation signature, a pattern of chemical marks on their DNA that differs from non-twins. This epigenetic signature appears in adult tissues and spans regions involved in cell adhesion, early cell-fate decisions, and structures near chromosome tips and centers.8Nature Communications. Identical twins carry a persistent epigenetic signature of early genome programming Whether this signature is a cause of the embryo splitting or merely a consequence of it remains unclear, but its existence suggests the event isn’t quite as random as once believed. Still, no one has found a gene for identical twinning the way they’ve found FSHB for fraternal twinning, so you can’t meaningfully predict identical twins from family history.
Body Size, Age, and Other Factors Beyond Genetics
Genes aren’t the whole story even for fraternal twinning. A woman’s body mass index and height both independently affect her chances. A study of U.S. birth records found that women with a BMI of 30 or higher had significantly elevated odds of fraternal twinning compared to leaner women, and taller women also had higher odds, though the height effect was smaller in magnitude. Importantly, BMI was not linked to identical twinning, only fraternal, which makes sense because the connection presumably runs through the same hormonal pathways that govern ovulation.9PubMed. Relationship of maternal body mass index and height to twinning
Maternal age is another strong factor. FSH levels tend to rise as a woman approaches her late 30s and early 40s, because the ovaries become less responsive and the pituitary gland compensates by pumping out more FSH. That hormonal surge can cause multiple follicles to mature simultaneously. This is one reason fraternal twin rates climb with maternal age, peaking in the late 30s, even without fertility treatments. Parity matters too: women who have already had several pregnancies have slightly higher twinning rates, likely reflecting both hormonal shifts and a selection effect where more fertile women end up with more pregnancies overall.
Why Twinning Rates Vary So Dramatically Around the World
Fraternal twinning rates range from roughly 1% to 4% of births depending on the population, and nearly all of that global variation is due to differences in fraternal rather than identical twinning.5PubMed Central. Twinning across the Developing World Central West Africa has some of the highest natural fraternal twin rates in the world, while parts of East and Southeast Asia have some of the lowest. European populations fall somewhere in between.
These patterns are consistent with a genetic explanation: populations where the hyperovulation-associated gene variants are more common will naturally produce more fraternal twins. But nutrition, average maternal body size, and cultural factors like age at childbearing all layer on top of the genetics. Disentangling how much of the cross-country variation comes from gene frequency differences versus diet and demographics has proven difficult. What’s clear is that the variation is real and longstanding, and it tracks with fraternal rather than identical twinning, reinforcing the idea that fraternal twinning is the heritable type.
Evolutionary Pressures on Twinning
If carrying genes for fraternal twinning also comes with a package of broader fertility traits, as the FSHB data suggests, then an interesting evolutionary question arises: why aren’t those genes even more common? A recent genome-wide study found signals of long-term natural selection against the tendency to fraternal twinning, consistent with a broader mammalian trend toward smaller litter sizes.10Human Reproduction. Genome-wide association study meta-analysis of dizygotic twinning illuminates genetic regulation of female fecundity
Twin pregnancies carry real risks: preeclampsia, preterm birth, and low birth weight are all more common. Throughout most of human history, delivering twins with limited medical care was dangerous for both mother and babies. So while genes that boost ovulation may help a woman conceive, the downstream costs of twin pregnancy appear to have kept those genes from spreading further. The result is a balance: hyperovulation variants persist in the population because they confer some fertility advantages, but they remain at moderate frequencies because twinning itself carries penalties.
Assisted Reproduction and the Twin Boom
Any discussion of twin frequency today has to acknowledge that a substantial fraction of twin births, particularly in high-income countries, result from fertility treatments rather than spontaneous twinning. In vitro fertilization with multiple embryo transfers and ovulation-stimulating drugs both sharply increase the chance of fraternal twins. These twins arise from an artificial version of the same mechanism: multiple eggs being available for fertilization at once. The genetics of the mother play little role in this scenario because the drugs override her natural hormone levels.
This matters when you’re looking at your own family history. If a relative had twins after fertility treatment, that tells you nothing about inherited twinning genes. Only spontaneous twins, those conceived without medical assistance, are informative about whether hyperovulation runs in the family. Researchers studying the genetics of twinning take care to exclude IVF and drug-assisted conceptions from their analyses for exactly this reason.3PubMed Central. Identification of Common Genetic Variants Influencing Spontaneous Dizygotic Twinning and Female Fertility
Vanishing Twins and Hidden Conceptions
One wrinkle people rarely consider is that twin conceptions are more common than twin births. Vanishing twin syndrome, where one twin is lost early in the first trimester, occurs in an estimated 15 to 35% of twin pregnancies.11Elsevier / ScienceDirect (Best Practice & Research Clinical Obstetrics & Gynaecology). The vanishing twin: Diagnosis and implications Before routine early ultrasounds, most of these losses went entirely undetected. A woman might carry genetic variants that cause her to ovulate twice regularly, conceive twins fairly often, and yet never deliver a pair because one embryo fails early in development.
This has implications for how we interpret family histories. A family line might carry strong hyperovulation genetics without a single recorded set of twins, simply because the second embryo didn’t survive in enough cases. It also means that if you’re a singleton who was identified as a twin on an early ultrasound before one gestational sac disappeared, the twinning genes in your family did their job; it was the pregnancy, not the genetics, that didn’t follow through.
What Animal Breeding Has Revealed
Livestock genetics offers a useful window into twinning biology because breeders have spent decades selecting for higher litter sizes. In sheep, for example, specific genes that increase the number of ovulations per cycle have been identified and bred for commercially. A study of Katahdin ewes found candidate genes for fecundity including CNOT11, GLUD1, and several others not previously linked to fertility in sheep but identified in cows, sows, and buffalo.12PubMed Central. Detection of Candidate Genes Associated with Fecundity through Genome-Wide Selection Signatures of Katahdin ewes The takeaway for humans isn’t that we share the exact same genes, but that across mammalian species, the genetic architecture of multiple ovulation involves many genes of small effect rather than a single “twin gene.” Human fraternal twinning almost certainly works the same way: FSHB and SMAD3 are the first variants found, not the only ones operating.
How to Read Your Own Family Tree
If you’re trying to figure out whether you’re likely to have twins, here’s what actually matters. Look at the women on your mother’s side. If your mother, maternal grandmother, or maternal aunts had spontaneous fraternal twins, you’re more likely to carry hyperovulation variants yourself. Twins on your father’s side don’t increase your risk directly, but if you’re a woman whose father came from a twin-rich family, you may have inherited those variants from him; they just couldn’t express themselves until they landed in your body. If you’re a man wondering whether your children will include twins, your own genetics are mostly irrelevant. What matters is your partner’s genetics and her family history.
Identical twins in the family don’t meaningfully change anyone’s odds. Since identical twinning doesn’t follow a clear inheritance pattern and happens at roughly the same rate everywhere, a relative who had identical twins is essentially a coincidence from a genetic-risk standpoint. Many families conflate identical and fraternal twins in their oral histories, so if possible, it helps to know which type the family twins actually were before drawing conclusions about what it means for you.
Worth keeping in mind: even a woman who carries every known risk variant and checks every demographic box still has a low absolute chance of fraternal twins in any given pregnancy. The baseline rate for spontaneous fraternal twins is roughly 1 to 4% depending on population, and genetic variants push that probability up by modest increments. An 18% relative increase per gene copy sounds impressive until you remember it’s 18% of a small number. Having twins remains an uncommon event for any individual pregnancy, and the genetics are probabilistic, not deterministic.