Fraternal twins are overwhelmingly a maternal-side phenomenon, driven by the mother’s tendency to release more than one egg during ovulation. The father’s genes do not directly cause double ovulation, for the obvious reason that fathers do not ovulate. But the full picture is more interesting than that clean division suggests, because research has found that the paternal side is not entirely irrelevant, and identical twinning follows different rules altogether.
Why Fraternal Twinning Traces Through Mothers
Fraternal (dizygotic) twins happen when two eggs are released during a single cycle and both get fertilized. The key biological driver is follicle-stimulating hormone, or FSH, which triggers the ovaries to mature and release eggs. Women who naturally produce higher levels of FSH, or who produce it in more pronounced pulses, are more likely to release two eggs at once. Research has confirmed that mothers of hereditary dizygotic twins have higher and more pulsatile FSH levels than other women.1PubMed. Increased levels and pulsatility of follicle-stimulating hormone in mothers of hereditary dizygotic twins This hormonal tendency can be inherited, which is why fraternal twins appear to “run in families” on the mother’s side. A woman whose mother or grandmother had fraternal twins may have inherited gene variants that push her own FSH levels higher, making double ovulation more likely.
Because twinning depends on what the mother’s ovaries do, a father cannot directly make fraternal twins happen. His sperm fertilizes whatever eggs are available, whether that is one or two. The common belief that “twins skip a generation” is a garbled version of this reality. What actually happens is that a man can carry gene variants associated with higher FSH without those variants doing anything in his own body. If he passes them to a daughter, she may produce the elevated FSH that leads to double ovulation. So from the outside, it looks like twinning skipped a generation, when really it just passed through a generation that could not express the trait.
What Fathers Actually Contribute
Saying “twins run on the mother’s side” is the right general answer, but several studies have found that fathers are not entirely bystanders. A large analysis of families with nearly a thousand twin pairs found a paternal role in dizygotic twinning, though not in identical twinning.2Nature. Familial incidence of twinning The mechanism is not fully understood, but there are a few plausible explanations.
One involves the gene-carrier route described above. A man who inherited variants near genes like FSHB from his mother can pass those same variants to his daughters, who then have a higher chance of releasing two eggs. In this scenario, the father contributes indirectly by being a genetic middleman. His DNA carries the blueprint for elevated FSH production; it just does not switch on in his body.
Another possibility involves sperm quality. A Danish study found that fathers of both fraternal and identical twins had sperm with better morphology and higher motility than fathers who had singletons. Fathers of dizygotic twins had roughly 11.5 percentage points higher motile sperm, and fathers of identical twins showed a similar pattern.3PubMed. Twin pregnancy possibly associated with high semen quality The idea is that when two eggs are released, higher-quality sperm may be more likely to successfully fertilize both of them. This does not cause double ovulation, but it could raise the odds that double ovulation results in a twin pregnancy rather than a single pregnancy with one unfertilized egg.
Paternal age also seems to play a small independent role. A study based on birth records in Jerusalem found that after controlling for maternal age, fathers aged 35 and older still had about 50 percent higher odds of a twin delivery compared to fathers under 25.4PubMed Central. Paternal age and twinning in the Jerusalem Perinatal Study The researchers noted that part of what is usually attributed to maternal age may actually be confounded by paternal age, since older mothers tend to have older partners. The biological mechanism behind this association remains unclear.
The Genes Behind Double Ovulation
Researchers have identified specific gene variants that increase the likelihood of fraternal twinning, and the findings reinforce why this trait is primarily maternal. The first large genome-wide association study of spontaneous dizygotic twinning pinpointed two significant variants: one near FSHB on chromosome 11, and another in SMAD3 on chromosome 15.5American Journal of Human Genetics. Genome-wide Association Analyses Identify Maternal Loci Influencing Spontaneous Dizygotic Twinning and Fertility The FSHB variant is the more intuitive one: it sits right next to the gene that encodes the beta subunit of FSH itself. Women who carry this variant appear to produce FSH in a way that favors releasing multiple eggs.
A larger follow-up meta-analysis confirmed those two signals and added two more, near genes called DOCK5 (adjacent to GNRH1, which helps regulate the hormonal cascade triggering ovulation) and ZFPM1.6PubMed Central. Genome-wide association study meta-analysis of dizygotic twinning illuminates genetic regulation of female fecundity All four loci are involved in the hormonal signaling that controls the ovarian cycle. None of them have anything to do with what happens on the sperm side. A father can carry these variants and pass them along, but they only influence twinning when they land in a daughter who has ovaries to respond to the hormonal signals.
These gene variants each have a modest individual effect, raising the odds of fraternal twinning by roughly 7 to 40 percent depending on the variant. That matters because it means no single “twin gene” determines the outcome. Fraternal twinning is influenced by multiple genetic factors acting together, layered on top of age, body composition, and other variables. A woman could carry one variant and never have twins, or carry several and still not twin, because the system depends on many moving parts.
Identical Twins Follow Different Rules
The question “do twins run in families?” usually gets answered with a focus on fraternal twins, because the genetics there are better understood. Identical (monozygotic) twins, which form when a single fertilized egg splits into two embryos, have traditionally been considered a random event with no genetic basis. The worldwide rate of identical twinning has historically been relatively stable at about 4 per 1,000 live births, regardless of ethnicity or geography, which supported the idea that it was not hereditary.7PubMed Central. Adverse pregnancy, birth, and infant outcomes in twins: Effects of maternal fertility status and infant gender combinations
That picture is shifting. Familial clustering of identical twins turns out to be more common than researchers used to think. A review of the evidence noted that familial monozygotic twinning is more frequently reported than the older literature suggested, with multiple families showing repeated identical twinning across generations.8PubMed Central. Non-identical monozygotic twins, intermediate twin types, zygosity testing, and the non-random nature of monozygotic twinning: a review And the large family study from Nature found evidence that a propensity toward identical twinning can be inherited through the maternal line, even raising the possibility that the mechanisms behind identical and fraternal twinning may be related.2Nature. Familial incidence of twinning That same study, however, did not find a paternal role in identical twinning, which lines up with the idea that whatever causes an embryo to split is happening after fertilization and is governed by the embryo’s early developmental program rather than by anything the sperm contributed.
A striking discovery came from research showing that identical twins carry a lifelong molecular signature in their DNA. Specifically, identical twins share a distinctive pattern of DNA methylation, a chemical modification that affects how genes are read. This signature spans regions involved in cell adhesion, cell fate, and other processes relevant to early embryonic development.9PubMed Central. Identical twins carry a persistent epigenetic signature of early genome programming The researchers demonstrated that this signature is stable enough to retrospectively identify whether an individual was conceived as an identical twin, even if they lost their co-twin early in pregnancy. This finding suggests that whatever triggers embryo splitting is not random noise but a reproducible biological process, which opens the door to the idea that it could be heritable after all.
Age, Body Size, and Other Factors That Shift the Odds
Genetics is only part of the story for fraternal twins. Maternal age is one of the strongest non-genetic predictors. Women in their mid-to-late thirties are more likely to release multiple eggs per cycle than women in their twenties, likely because of rising FSH levels as the body works harder to stimulate the ovaries in the years approaching menopause. This does not mean older mothers are “genetically” predisposed; it means the same hormonal pathway that genes influence is also influenced by age. Research into the evolutionary logic of this pattern has concluded that double ovulation is an adaptive strategy tied to aging: as the chance of any single embryo surviving declines with age, the body hedges its bets by releasing two eggs.10Nature Ecology & Evolution. An age-dependent ovulatory strategy explains the evolution of dizygotic twinning in humans The researchers argued that the most successful ovulatory strategy would be to always double ovulate as insurance against early fetal loss, but ideally never bear twins, since twin pregnancies carry higher risks. Twinning, in this view, is a side effect of a fertility-preserving strategy, not something that was selected for on its own.
Body size also matters. Women with a BMI of 30 or higher have significantly elevated odds of dizygotic twinning compared to leaner women, and the risk rises in a dose-response pattern as BMI increases.11Obstetrics & Gynecology. Relationship of Maternal Body Mass Index and Height to Twinning Taller women also appear to have slightly higher rates. The suspected link is insulin-like growth factor, which is higher in taller and heavier women and may stimulate the ovaries alongside FSH. None of these factors involve the father’s genes.
Population Differences and Their Genetic Roots
Fraternal twinning rates vary enormously across populations, which further underscores the genetic component. Sub-Saharan African populations have the highest natural rates of dizygotic twinning in the world. In The Gambia, fraternal twins account for up to 3 percent of live births, roughly three to four times the rate seen in many European or Asian populations.12PubMed Central. PTX3 genetic variation and dizygotic twinning in the Gambia: could pleiotropy with innate immunity explain common dizygotic twinning in Africa? East Asian populations tend to have the lowest rates. European populations fall in between.
These differences are too consistent and large to be explained by diet or climate alone, and they persist even when researchers control for maternal age and body size. The Gambian study investigated a particularly interesting gene called PTX3, which encodes a protein involved in both innate immunity and female fertility. The researchers found that certain PTX3 variants were more common in Gambian women who had dizygotic twins, and that these same variants overlapped with variants previously linked to resistance to tuberculosis. The implication is that natural selection for immune defense against infectious disease may have inadvertently pushed up the twinning rate in populations where those infections were common. Genes do not always do just one thing, and a variant that protects against tuberculosis can also affect how the ovaries respond during a cycle.
Assisted Reproduction Has Changed the Landscape
Any conversation about twinning rates today has to acknowledge the massive impact of fertility treatments. In the United States, the overall twinning rate roughly doubled between 1980 and 2013, driven in large part by assisted reproductive technologies and ovulation-stimulating medications. The rate of identical twinning has also risen with assisted conception, with estimates suggesting it more than doubled from about 4 per 1,000 to 8 or 9 per 1,000 live births in assisted pregnancies.7PubMed Central. Adverse pregnancy, birth, and infant outcomes in twins: Effects of maternal fertility status and infant gender combinations This means that many families today who have twins are not experiencing a genetically inherited tendency at all. When someone says “twins run in my family,” it is worth considering whether recent twins in the family tree were conceived with fertility assistance, because that changes the meaning of the pattern entirely.
Fertility medications like clomiphene work by stimulating the same FSH pathway that genetic variants naturally boost, effectively pharmacologically mimicking the “twin-prone” hormonal profile. And when clinics transfer multiple embryos during IVF, fraternal twins result not from any inherited tendency but from a clinical decision. The recent trend toward single-embryo transfer in many countries has started to bring twin rates back down, separating out the genetic signal from the medical one.
Sesquizygotic Twins and the Blurry Space Between
Most discussions of twin genetics assume a clean binary: identical twins from one egg, fraternal twins from two. But a rare intermediate category exists. In 2019, researchers published findings on sesquizygotic twins, who are genetically somewhere between identical and fraternal. These twins were maternally identical but shared only about 78 percent of their paternal genome, indicating that a single egg was fertilized by two sperm and then divided into two embryos with different mixtures of paternal DNA.13PubMed Central / New England Journal of Medicine. Molecular Support for Heterogonesis Resulting in Sesquizygotic Twinning This is an extremely rare event, but it demonstrates that twinning mechanisms are not as neatly categorized as textbooks suggest.
In the sesquizygotic case, the paternal contribution actually matters in a unique way: it is the presence of two different sperm that creates the unusual genetic makeup. This is not evidence that paternal genes “cause” twinning in any heritable sense, but it does show that the father’s gametes can be mechanistically central to certain kinds of twin outcomes. The condition was identified through detailed genetic analysis that would not have been performed in a standard pregnancy, which raises the possibility that sesquizygotic twins are slightly less rare than the single published case implies, though they are still presumed to be extraordinarily uncommon.
What “Runs in the Family” Actually Means
When families ask whether twins are hereditary, they usually want a practical answer about their own chances. For fraternal twins, your family history on the mother’s side is the most informative signal. A woman whose mother, maternal grandmother, or maternal aunt had spontaneous fraternal twins has a meaningfully elevated chance of releasing two eggs herself. The gene variants involved are common enough that this kind of family clustering is not unusual. If your father’s side of the family has fraternal twins, that is less directly informative for you if you are a woman, but it is not irrelevant: your father may have passed you the same FSHB or SMAD3 variants that made his mother or sisters prone to double ovulation. The claim that “twins only run on the mother’s side” oversimplifies the genetics, though it correctly identifies which side matters most.
For identical twins, the honest answer is that we still do not have a clean genetic predictor. The familial clustering and the epigenetic signature research point toward some heritable component, but the specific genes involved have not been identified the way they have for fraternal twinning. If identical twins appear repeatedly in your family, that could be meaningful, but the science is not yet advanced enough to quantify the risk. And because identical twinning happens after fertilization, neither the paternal nor the maternal side has an obvious monopoly on whatever underlying biology is involved, though the available family data lean maternal.
One practical note: the “twins skip a generation” folklore is not a biological rule. It is a pattern that sometimes emerges because a father can silently carry twin-prone gene variants without expressing them, and his daughter can then express them. But twins can also appear in consecutive generations on the maternal side, because a mother who has fraternal twins may pass the same gene variants directly to her daughter. The skipping pattern is one possible outcome, not a law of inheritance.