Fraternal twins run in families primarily through the mother’s side, because the key biological event, releasing two eggs in a single cycle, depends on the mother’s ovulation biology. Identical twins, by contrast, appear to be mostly a random embryological event with no clear inheritance pattern from either parent. That clean split is the short answer, but the fuller picture is more interesting, touching on specific genes, the father’s surprising supporting role, and some genuinely strange edge cases.
Why Fraternal Twins Depend on the Mother
Fraternal (dizygotic) twins happen when two separate eggs are fertilized by two separate sperm in the same cycle. The rate-limiting step is hyperovulation, the release of more than one egg. Since ovulation is something the mother’s body does, the genetic tendency toward it is carried and expressed by the mother. A woman who inherits variants that make her more likely to release multiple eggs per cycle is more likely to conceive fraternal twins, regardless of who the father is.
A large genome-wide study pinpointed two key genetic regions linked to dizygotic twinning. The strongest signal sat near the gene FSHB, which encodes part of follicle-stimulating hormone, a hormone that drives the maturation of eggs in the ovaries. The variant associated with twinning also raised circulating FSH levels, and the same version of the gene that boosted FSH the most was the version that most increased the chance of fraternal twins.1American Journal of Human Genetics. Genome-wide Association Meta-analysis Identifies Susceptibility Loci for Dizygotic Twinning More FSH means more follicles maturing at once, which means a higher chance of dropping two eggs instead of one. This is why the “twins run in the family” observation tracks through mothers, grandmothers, and maternal aunts rather than through the father’s line. A man can carry the hyperovulation variant and pass it to his daughter, who might then have fraternal twins herself, but the gene does nothing in his own body. This is the grain of truth behind the folk saying that “twins skip a generation,” though the real genetics are messier than that simple rule implies.
The Father’s Role Is Smaller but Not Zero
Fathers are often written out of the twinning story entirely, which is slightly unfair. One Danish study compared semen quality among fathers of twins versus fathers of singletons. Fathers of both fraternal and identical twins had a higher percentage of sperm with normal shape and better motility than the reference group. Fathers of fraternal twins had roughly 11 to 12 percentage points more motile sperm, and the trend for higher sperm concentration also pointed upward, though it did not reach statistical significance for concentration alone.2PubMed. Twin pregnancy possibly associated with high semen quality
This does not mean that a man’s sperm “causes” twins. The interpretation is subtler: if two eggs are available, higher-quality sperm might be more likely to fertilize both of them successfully. So the father’s contribution is more of an enabling factor than a driving one. You still need the mother to ovulate twice. But it suggests that twinning is not purely a maternal event in the way it is sometimes portrayed.
Identical Twins Are Mostly a Roll of the Dice
Identical (monozygotic) twins come from a single fertilized egg that splits into two embryos. This happens in roughly 3 to 4 out of every 1,000 births worldwide, and unlike fraternal twinning, the rate is remarkably stable across populations, maternal ages, and family histories. That stability is one reason scientists have long suspected the splitting is essentially random rather than genetically programmed.
No one has found a gene that clearly increases the chance of an embryo splitting. The leading hypothesis involves the zona pellucida, the protective shell around the early embryo. As the growing cluster of cells, called a blastocyst, pushes its way out of this shell in a process called hatching, the inner cell mass can occasionally divide.3Human Reproduction. P-222 Impact of zona pellucida opening size using laser-assisted hatching on zygotic splitting in 9,425 pregnancies after frozen-thawed single blastocyst transfer Early IVF research noticed that pregnancies from embryos with naturally thin shells, or embryos whose shells had been deliberately breached during assisted hatching, were more likely to produce identical twins.4Human Reproduction. Fertilization and early embrology: Monozygotic twinning in the human is associated with the zona pellucida architecture This suggests a mechanical trigger rather than a genetic instruction from either parent.
That said, identical twins do carry a distinctive biological signature. A large epigenetic study across multiple twin registries found 834 sites in the genome where chemical tags on DNA differed consistently between identical and fraternal twins. These tags were present from early life and remained stable over time, pointing to something that happens around the moment of embryo splitting rather than afterward.5Nature Communications. Identical twins carry a persistent epigenetic signature of early genome programming Whether those epigenetic marks are a cause of the split or a consequence of it is still an open question. Either way, they are not inherited from the mother or father in the usual sense. They appear to be set during the embryo’s earliest cell divisions.
What “Twins Skip a Generation” Actually Means
This is one of the most persistent pieces of twin folklore, and it is partly right for the wrong reasons. The idea is that if your grandmother had fraternal twins, you are more likely to have them than your mother was. The reality is simpler and less mystical: the hyperovulation gene can be passed by either parent, but it only shows its effect in a body that ovulates. A man who inherits the variant from his twin mother will not have twins himself, because he does not ovulate. But he can pass the variant to his daughter, who can. From the outside, it looks like the trait skipped his generation. In families where the gene passes from mother to daughter directly, there is no skip at all.
The “skipping” pattern also does not apply to identical twins. Since identical twinning does not appear to be driven by inherited genes in a straightforward way, having an identical twin in the family does not meaningfully change anyone else’s odds.
Maternal Body Size and Twinning Rates
Beyond genetics, certain physical characteristics of the mother are associated with fraternal twinning. A study examining birth records found that women with a BMI of 30 or higher had significantly elevated odds of fraternal twins, and the trend of increasing risk with increasing BMI was strong. Taller women also showed higher odds of fraternal twin pregnancies, though the effect was smaller than the one linked to BMI. Neither height nor BMI was connected to identical twinning rates.6Obstetrics & Gynecology. Relationship of Maternal Body Mass Index and Height to Twinning
The biological explanation likely circles back to hormones. Higher body mass is associated with higher circulating levels of insulin-like growth factor (IGF-1), which can stimulate the ovaries independently of FSH. Taller women also tend to have higher IGF-1 levels. The pattern reinforces the broader point: fraternal twinning is gated by the mother’s ovulatory biology, and anything that nudges that biology toward releasing more eggs, whether it is a gene variant or a hormonal milieu shaped by body composition, pushes the odds up.
Population Differences Tell the Same Story
Some of the most striking evidence that fraternal twinning is a maternal-side trait comes from comparing populations. The Yoruba people of Nigeria have a twinning rate more than four times that of European populations, and research has shown this is driven by higher rates of dizygotic (fraternal) twins specifically, not identical ones.7PubMed. The Yoruba contribution to our understanding of the twinning process Meanwhile, East Asian populations have some of the lowest fraternal twinning rates globally. Identical twinning rates, by contrast, barely budge between these groups. The variation in fraternal rates across populations maps onto differences in maternal ovulatory patterns and FSH levels, not onto anything about paternal genetics.
There has been debate over whether the elevated Yoruba rate is entirely genetic or whether environmental factors, possibly dietary, play a role as well. Either way, the variation sits squarely on the maternal side of the equation.
How IVF Changes the Equation
Fertility treatments complicate the clean “mom’s side for fraternal, random for identical” framework. Ovulation-stimulating drugs increase fraternal twinning for the obvious reason: they push the ovaries to release multiple eggs. But IVF also raises the rate of identical twinning, which is more surprising. A multicenter study found that about 1.2% of pregnancies from single embryo transfer (where only one embryo is placed in the uterus) resulted in identical twins. That is several times the natural rate.8PubMed. Risk Factors for Monozygotic Twins in IVF-ICSI Cycles: a Case-Control Study
The strongest risk factor for identical twinning after IVF was assisted hatching, a technique where the embryo’s outer shell is deliberately thinned or punctured to help it implant. After adjusting for other variables, assisted hatching roughly doubled the odds of the embryo splitting. Extended culture to the blastocyst stage before transfer has also been linked to higher identical twinning rates.9Human Reproduction. Monozygotic twins and transfer at the blastocyst stage after ICSI Both findings point back to the zona pellucida mechanism: physically manipulating the shell or culturing the embryo longer may make the cell mass more prone to dividing as it hatches. In IVF, then, the cause of identical twinning is neither mom nor dad. It is the lab procedure itself.
Mothers of Twins Tend to Be Unusually Robust
From an evolutionary standpoint, carrying and raising two babies at once is enormously costly. Research using large historical birth records has found that mothers who bore twins also had shorter intervals between all their births, had their last baby at an older age, and achieved higher lifetime fertility than mothers who only had singletons. They also showed lower postmenopausal mortality. The researchers concluded that twinning is not a random misfire of the reproductive system; instead, it tends to happen to women with a “robust phenotype” who can handle the extra demands.10PubMed Central. Twinning in humans: maternal heterogeneity in reproduction and survival
This reframes the question nicely. The genes that increase fraternal twinning are not random quirks. They appear to be part of a broader maternal phenotype linked to high fertility and physical resilience. A woman who twins is not just someone who happened to release two eggs. She is, on average, someone whose entire reproductive profile runs hotter, and that profile is shaped by her own genetic makeup.
When Twins Have Different Fathers
In rare cases, the father’s identity is not just secondary but split. Heteropaternal superfecundation occurs when two eggs released in the same cycle are fertilized by sperm from different men during separate acts of intercourse.11PubMed Central. Twins from different fathers: A heteropaternal superfecundation case report in Colombia The resulting twins are genetically half-siblings, sharing about 25% of their variable DNA rather than the roughly 50% that ordinary fraternal twins share.12PubMed. Heteropaternal twinning: Unique case of opposite-sex twins with different fathers
Cases are identified through paternity testing, sometimes prompted by the twins looking strikingly different from each other. One forensic genetics case used advanced DNA sequencing of cell-free DNA from the mother’s blood plasma to identify heteropaternal superfecundation as early as ten weeks of pregnancy.13Forensic Science International: Genetics. A case of heteropaternal superfecundation identified by microhap sequencing of maternal plasma cell-free DNA Heteropaternal superfecundation is extremely rare, but it makes a vivid point: fraternal twinning starts with the mother ovulating twice, and only after that do the fathers enter the picture.
Sesquizygotic Twins and Other Oddities
Somewhere between identical and fraternal sits a vanishingly rare category that was only confirmed genetically in 2019. Sesquizygotic twins appear to start from a single egg fertilized by two sperm simultaneously, with the resulting cell mass dividing into two embryos that are identical on the mother’s side but share only a portion of their paternal DNA. The first confirmed case involved twins who were maternally identical but shared 78% of their father’s genome, making them genetically intermediate between identical and fraternal.14PubMed. Molecular Support for Heterogonesis Resulting in Sesquizygotic Twinning Only a handful of cases have ever been documented. The existence of sesquizygotic twins is a reminder that the categories of “identical” and “fraternal” are convenient labels for the two common outcomes, but the biology of early embryo development can occasionally produce something in between.
The Vanishing Twin
One underappreciated aspect of twinning is how often it begins but does not make it to delivery. With routine early ultrasound, clinicians noticed that twin pregnancies identified in the first trimester sometimes became singleton pregnancies by the second. The lost twin is resorbed by the mother’s body without any visible miscarriage. Studies have documented “disappearance” rates ranging widely, but the phenomenon occurs in a substantial share of early twin pregnancies.15Acta geneticae medicae et gemellologiae: twin research. The Vanishing Twin More recent estimates place the prevalence of vanishing twin syndrome at roughly 15 to 35% of twin pregnancies.16Best Practice & Research Clinical Obstetrics & Gynaecology. The vanishing twin: Diagnosis and implications
This means twinning, as a biological event, is far more common than birth statistics suggest. Many people walking around as apparent singletons began as twins in the first weeks of development. The vanishing twin phenomenon applies equally to conceptions influenced by maternal hyperovulation genes and to random embryo splits, so it does not change the answer about which parent “causes” twins. But it does change how common twinning actually is in human reproduction. The genetic and mechanical triggers for twinning fire more often than anyone realized before ultrasound made it visible.
An Armadillo, for Perspective
Humans are not the only mammals that produce identical multiples, and the most instructive comparison is the nine-banded armadillo, which routinely gives birth to identical quadruplets from a single fertilized egg. Research has noted that human fertilized eggs appear capable of undergoing the same kind of early cell-mass divisions that armadillo embryos do, and can give rise to various combinations of identical multiples.17PubMed. On the possible cause of monozygotic twinning: lessons from the 9-banded armadillo and from assisted reproduction In the armadillo, identical splitting is the default, not a freak event. In humans, the same basic cellular machinery exists, but it fires only rarely. Why human embryos split occasionally while armadillo embryos split every time remains one of the genuinely open questions in reproductive biology. It is another piece of evidence, though, that identical twinning is driven by events in the embryo itself rather than by something either parent contributes genetically.
Identical Twins Diverge Over a Lifetime
Even when identical twins start from the same DNA, they do not stay molecularly identical. A study of monozygotic twin pairs across different ages found that young twins were epigenetically indistinguishable, but older pairs showed large differences in how their genes were chemically tagged and expressed.18PubMed Central. Epigenetic differences arise during the lifetime of monozygotic twins These divergences accumulate through different environmental exposures, diets, and life experiences. The finding is relevant here because it undercuts the idea that identical twins are carbon copies determined entirely by their shared parental DNA. Both the initial splitting event and the subsequent divergence involve mechanisms that neither parent directly controls.
Some of these epigenetic differences are also present from birth. The large meta-analysis that identified 834 sites of DNA methylation difference between identical and fraternal twins found that these marks were set very early, likely around the time the embryo split.19Human Reproduction. New insights into the (epi)genetics of twinning So identical twins are stamped by their twinning event itself, in ways that persist for life, and those stamps are neither maternal nor paternal in origin. They arise from whatever happens during that early embryonic division.