For fraternal twins, the mother’s biology is the primary driver. Fraternal twins happen when two eggs are released in a single cycle, and whether a woman’s ovaries do this depends largely on her own genetics, hormonal profile, age, and body composition. The father contributes sperm, but he has no direct influence over how many eggs his partner ovulates. Identical twins are a different story: the splitting of a single fertilized embryo into two appears to be a mostly random event that neither parent reliably controls, though emerging research on epigenetics is complicating that picture.
How Fraternal Twins Actually Happen
Fraternal twins, also called dizygotic twins, result from a woman releasing two eggs during the same ovulatory cycle, a phenomenon known as hyper-ovulation. Each egg is fertilized by a separate sperm cell, producing two genetically distinct siblings who share roughly half their DNA, the same as any pair of siblings born at different times.1PubMed Central. Twin Study Design Because the key event is the release of two eggs, the mother’s reproductive system is doing the work that makes twinning possible. Natural dizygotic twinning is considered a marker of female fertility.2PubMed Central. Polygenic scores for dizygotic twinning: insights into the genetic architecture of female fertility
This is why the “twins run in the family” saying has a specific meaning that most people get slightly wrong. Twins run on the mother’s side of the family, because it is the mother’s tendency toward hyper-ovulation that gets inherited. If a man’s mother or sister had fraternal twins, that tells you something about the women in his family, not about his ability to “cause” twins. He could carry gene variants associated with hyper-ovulation and pass them to a daughter, who might then have a higher chance of conceiving fraternal twins herself. But those genes do nothing in his own body during conception.
The Specific Genes Behind Hyper-Ovulation
Researchers have identified specific genetic variants linked to spontaneous fraternal twinning. A large study pinpointed two variants: one near the FSHB gene and another within the SMAD3 gene. FSHB is involved in producing follicle-stimulating hormone, which triggers egg development in the ovaries. The variant near FSHB increased a mother’s chance of delivering twins by about 18% per copy of the risk allele, while the SMAD3 variant increased the chance by about 9% per copy.3PubMed Central. Identification of Common Genetic Variants Influencing Spontaneous Dizygotic Twinning and Female Fertility These are modest bumps in probability, not guarantees. A woman could carry both variants and never have twins, or a woman with neither variant could still release two eggs in a cycle.
The FSHB variant was also associated with higher blood levels of follicle-stimulating hormone, earlier puberty, and higher lifetime number of pregnancies, all of which reinforce the connection between the gene, ovulation intensity, and fertility in general.3PubMed Central. Identification of Common Genetic Variants Influencing Spontaneous Dizygotic Twinning and Female Fertility The SMAD3 gene product plays a role in how the ovaries respond to FSH signals, essentially fine-tuning how many follicles mature to the point of releasing an egg. Similar pathways involving bone morphogenetic proteins regulate ovulation rate in other mammals.4PubMed Central. Regulation of ovulation rate in mammals: contribution of sheep genetic models These findings make clear that fraternal twinning is governed by the mother’s ovarian biology, with genetic variants she inherits from either of her own parents shaping her odds.
Identical Twins Are a Different Phenomenon Entirely
Identical twins, or monozygotic twins, form when a single fertilized egg splits into two embryos. Both embryos carry the same DNA because they came from one egg and one sperm.1PubMed Central. Twin Study Design Unlike fraternal twinning, there is no strong evidence that the tendency to have identical twins runs in families, and for decades the event was treated as essentially random. No ovulation quirk or sperm characteristic was known to reliably cause embryo splitting.
That view is shifting somewhat. A large epigenetic study found that identical twins carry a distinctive DNA methylation signature, a pattern of chemical marks on their genome that differs from those seen in fraternal twins. These marks clustered near telomeres and centromeres and in regions involved in cell adhesion and early cell-fate decisions.5Nature Communications. Identical twins carry a persistent epigenetic signature of early genome programming A follow-up meta-analysis of over 5,700 twins identified more than 800 sites in the genome where methylation levels differed between identical and fraternal twins.6Human Reproduction. New insights into the (epi)genetics of twinning
What this means is still being worked out. The signature could be a cause of embryo splitting, or it could be a consequence of it. If certain methylation patterns in the very early embryo predispose it to divide, then whoever contributed the epigenetic environment of that embryo, which is heavily influenced by the egg and its surrounding cellular machinery, would be the relevant parent. That again points more toward the mother’s biology, since the egg carries most of the early embryonic environment. But the research is young, and nobody yet claims to have identified a reliable maternal or paternal predictor for identical twinning.
Does the Father Contribute Anything to Twinning?
For fraternal twins, the father’s role is straightforward: he provides sperm, but whether one egg or two are waiting to be fertilized is entirely up to the mother’s ovaries. No genetic variant in the father has been shown to influence his partner’s ovulation rate. The “twins skip a generation” idea comes from a misunderstanding of this. A man whose mother had twins may carry a hyper-ovulation gene variant, but it only matters if he passes it to a daughter.
For identical twins, the picture is slightly more complicated but still doesn’t point to the father as a major player. One study on early embryo behavior found that the type of sperm used in fertility procedures could affect how embryos divide in their earliest stages. Embryos fertilized with surgically retrieved sperm had a higher rate of unusual cleavage patterns compared to embryos fertilized with ejaculated sperm.7PLOS ONE. Direct Unequal Cleavages: Embryo Developmental Competence, Genetic Constitution and Clinical Outcome Unusual cleavage is not the same as embryo splitting, but it demonstrates that sperm characteristics can at least influence early embryo behavior.
Research in animal models has also shown that seminal fluid, the liquid component of semen apart from the sperm cells themselves, affects the environment in the reproductive tract. In mouse studies, the absence of seminal fluid changed the expression of growth-supporting factors in the oviduct, which in turn affected embryo development and even the health of male offspring later in life.8PubMed Central. Maternal tract factors contribute to paternal seminal fluid impact on metabolic phenotype in offspring In humans, seminal plasma appears to help prime the uterine lining for implantation.9Human Reproduction Update. Contribution of semen to early embryo development: fertilization and beyond None of this has been directly connected to twinning. But it does mean the father’s contribution is more than just a DNA delivery: the composition of seminal fluid creates a biochemical context that shapes how the early embryo develops. Whether that context could ever nudge an embryo toward splitting remains speculative.
Maternal Age, Body Size, and Other Non-Genetic Factors
Genetics aside, several characteristics of the mother herself influence her odds of fraternal twins. Maternal age is one of the best-documented: the rate of fraternal twinning rises as women get older, peaking in the late 30s, before declining as fertility drops off. Researchers have proposed that this pattern reflects an adaptive strategy where the body shifts from releasing one egg per cycle to two as a kind of insurance against the increased rate of early pregnancy loss that comes with age. Twinning, under this model, is not selected for directly; it is a side effect of double ovulation, which evolved to improve the odds of at least one successful pregnancy.10Nature Ecology & Evolution. An age-dependent ovulatory strategy explains the evolution of dizygotic twinning in humans
Body size matters too. A large analysis found that women with higher body mass had progressively higher rates of twin pregnancies. Women with the highest levels of obesity had about 41% more twin deliveries compared to women of normal weight, while underweight women had roughly 16% fewer.11PubMed Central. Obesity, Twin Pregnancy, and the Role of Assisted Reproductive Technology An earlier study confirmed the pattern and added that taller women also had modestly higher odds of fraternal twins, though the height effect was smaller than the weight effect.12Obstetrics and Gynecology. Relationship of Maternal Body Mass Index and Height to Twinning Both studies noted that the relationship applied to fraternal twins specifically; identical twinning rates were not significantly related to body mass. The mechanism probably involves higher levels of insulin-like growth factor and other hormones in larger women, which could stimulate the ovaries to mature and release more than one egg.
Historical data adds another angle. In a study of pre-modern populations, mothers of twins showed lower mortality after menopause, shorter intervals between pregnancies, later last births, and higher total fertility than mothers who only had singletons.13PubMed Central. Twinning in humans: maternal heterogeneity in reproduction and survival The interpretation is that fraternal twinning tends to happen in women who are generally more robust and fertile, rather than being a random event that could happen to anyone equally.
Why Some Populations Have Far More Twins
Twin rates vary dramatically across the world, and the differences are overwhelmingly in fraternal twins. The Yoruba people of Nigeria have a twinning rate more than four times that of European populations, driven almost entirely by higher rates of fraternal and higher-order multiples.14PubMed. The Yoruba contribution to our understanding of the twinning process Whether this is purely genetic or partly environmental has been debated for decades. The town of Igbo-Ora in southwestern Nigeria is sometimes called the “twin capital of the world,” and local explanations range from diet to divine blessing. Researchers have considered biomedical factors including maternal age patterns, parity, possible genetic variants affecting gonadotropin hormones, and dietary phytoestrogens from yam-heavy diets.15PubMed. Genetics, Diet or Divine Blessing? Local and Scientific Explanations for High Dizygotic Twinning in Igbo-Ora, Nigeria
East Asian populations, by contrast, have historically had among the lowest natural fraternal twinning rates. These population-level differences reinforce the point that fraternal twinning is largely a maternal trait shaped by genetics and environment, since identical twinning rates are roughly constant across populations at about 3 to 4 per 1,000 births. If the father’s genetics played a direct role in causing twins, you would expect to see paternal ancestry influencing twin rates, and the data does not support that.
Fertility Treatments Change the Equation
The biggest modern factor in twin pregnancies is not genetics at all but assisted reproduction. Fertility treatments like ovulation-stimulating drugs and in vitro fertilization dramatically increase the rate of fraternal twins by encouraging the release of multiple eggs or by transferring more than one embryo. In these cases, the “cause” of twins is the medical intervention, not the biology of either parent.
More unexpectedly, fertility treatments also raise the rate of identical twins. A systematic review found that the overall incidence of identical twins after assisted conception was about 0.9%, compared to roughly 0.4% in natural conception. Blastocyst transfer, where the embryo is grown in the lab for five days before being placed in the uterus, was associated with about a four-fold increase in identical twinning. Intracytoplasmic sperm injection, where a single sperm is injected directly into the egg, was associated with about a two-fold increase.16Human Reproduction Update. The risk of monozygotic twins after assisted reproductive technology: a systematic review and meta-analysis The reasons are not fully understood. One hypothesis is that the mechanical manipulation of embryos or the culture conditions in the lab create slight disruptions in cell adhesion that make splitting more likely. If this is true, it suggests that identical twinning is sensitive to the early environment the embryo experiences, which is shaped by both laboratory conditions and the mother’s uterine environment after transfer.
The Rare Exception Where Both Parents Matter
There is one exotic type of twinning where the father’s contribution is unmistakably relevant. Sesquizygotic twins are genetically somewhere between identical and fraternal. In the only well-documented case, reported in 2019, the twins shared 100% of their maternal DNA but only about 78% of their paternal DNA, and they were of different sexes.17PubMed. Molecular Support for Heterogonesis Resulting in Sesquizygotic Twinning The suspected mechanism is that a single egg was fertilized by two different sperm cells simultaneously, and the resulting cell mass then divided into two embryos.18PubMed. The New Sesquizygotic Twins and More: Exotic Twin Types/Twin Research Reviews
This is extraordinarily rare, possibly a handful of cases in medical history. But it represents a scenario where the father’s contribution is structurally built into the type of twinning. Two separate sperm cells from the same ejaculate create the genetic chimerism that defines these twins. Whether anything about the father’s sperm makes this more likely is unknown; it could be pure chance at the moment of fertilization.
How Twin Type Gets Determined During Pregnancy
Because the question of whether twins are identical or fraternal matters for medical management, clinicians have traditionally used ultrasound to check whether twins share a placenta. Same-sex twins sharing a single placenta are almost certainly identical, but opposite-sex twins or same-sex twins with two placentas could be either type, and ultrasound alone cannot always tell. Newer blood-based screening tests analyze cell-free fetal DNA circulating in the mother’s blood. By looking at genetic markers from the fetuses, these tests can distinguish whether the two fetuses have different genomes (fraternal) or the same genome (identical).19PubMed Central. Clinician-reported chorionicity and zygosity assignment using single-nucleotide polymorphism-based cell-free DNA: Lessons learned from 55,344 twin pregnancies In a validation study, this approach correctly identified zygosity in all 93 twin pregnancies tested, and it can be done as early as nine weeks of gestation.20PubMed Central. Validation of a Single-Nucleotide Polymorphism-Based Non-Invasive Prenatal Test in Twin Gestations: Determination of Zygosity, Individual Fetal Sex, and Fetal Aneuploidy
This matters because identical twins who share a placenta face specific risks, including twin-to-twin transfusion syndrome, that fraternal twins do not. Getting the zygosity right early allows doctors to plan monitoring accordingly. It also resolves the family curiosity factor: parents of same-sex fraternal twins are often told their children “must be identical” by well-meaning relatives, and a definitive test can settle the question.
What About the “Twins Skip a Generation” Myth
The idea that twins skip a generation is one of the most persistent misconceptions in family folklore. It has a kernel of truth, but only in a narrow scenario. Imagine a woman who carries a hyper-ovulation gene variant and has fraternal twins. If she has a son, that son inherits the variant but, lacking ovaries, cannot express it. If that son has a daughter, the daughter may inherit the variant and, now having ovaries, may herself have twins. In that specific lineage, it looks like twins skipped the son’s generation. But if the original woman had a daughter instead of a son, the daughter could have twins in the very next generation, with no skipping at all.
The pattern only appears to skip when the relevant gene passes through a male. It is not a biological rule about twinning; it is an artifact of the fact that twinning genes only manifest in someone who ovulates. The same gene can produce twins in consecutive generations, or it can go silent for multiple generations if it keeps passing through men who never have daughters, or whose daughters happen not to express it strongly enough for double ovulation.
Identical twinning does not follow any known family pattern at all, so the “skipping a generation” concept does not apply there. And for both types of twins, plenty of twin births occur with no family history whatsoever, arising from age-related changes in ovulation, body composition factors, or simple chance.
When Fathers of Twins Have More Twins
Occasionally you hear about a man who has fathered twins with more than one partner, leading people to conclude that he must be “causing” the twins. This can happen, but the explanation is usually statistical rather than biological. A man who has children with several partners increases his chances of encountering a woman who happens to ovulate multiple eggs. If he comes from a family with high twinning rates, his partners may also come from overlapping population groups where hyper-ovulation genes are common. And in populations where assisted reproduction is widespread, a man involved in multiple IVF cycles may see twins appear more than once simply because of the treatment protocols used.
There is no identified mechanism by which something about a man’s sperm causes the woman’s body to release two eggs instead of one. The sperm arrives after ovulation has already occurred. By the time fertilization happens, the number of available eggs is already determined. The only paternal role that has any mechanistic foothold in twinning research is the extremely rare sesquizygotic scenario, and even there, the role is about what happens after fertilization, not about controlling egg release.