Twins run in families through genes that increase the chance of releasing more than one egg during ovulation. This genetic link applies only to fraternal twins, not identical twins. If your mother or grandmother had fraternal twins, you may have inherited a higher likelihood of the same, with the heritability of fraternal twinning estimated at roughly 26 to 28%.
Fraternal Twins Are the Ones That Run in Families
Fraternal twins form when two separate eggs are fertilized by two different sperm in the same cycle. For that to happen, a woman’s ovaries need to release more than one egg, a process called hyperovulation. Hyperovulation has a genetic component, which is why fraternal twins cluster in certain families.
Identical twins, by contrast, form when a single fertilized egg splits in two. Scientists generally consider this a random event with no clear genetic basis. So when people say “twins run in my family,” they’re really talking about fraternal twins, whether they realize it or not.
The Genes Behind Hyperovulation
Researchers have identified several genes involved in fraternal twinning, and they all relate to how the ovaries develop and release eggs. One key gene, FSHB, encodes part of follicle-stimulating hormone (FSH), the hormone that tells your ovaries to mature eggs each cycle. Certain variants of this gene are associated with higher FSH levels in the blood, which can push the ovaries to mature and release more than one egg.
Two other genes, GDF9 and BMP15, help regulate follicle growth and ovulation rates. They’re part of a signaling pathway that controls how many follicles develop to the point of releasing an egg. Variations in these genes can tip the balance toward releasing two eggs instead of one. The effect of any single gene is modest, but when several of these variants combine, the probability of fraternal twins rises meaningfully.
How the Genes Pass Through Families
Because hyperovulation only matters in people who ovulate, the genetic trait can seem to “skip” generations. Here’s how: a man can carry gene variants linked to hyperovulation and pass them to his daughter without ever showing any sign himself. His daughter then has a higher chance of releasing two eggs and conceiving fraternal twins. From the outside, it looks like twins skipped a generation, but the genes were quietly traveling through the father all along.
This means your family history on both sides matters. If your father is a fraternal twin, or if fraternal twins appear on his side of the family, you could still have inherited the relevant genes. The common belief that twins only come from the mother’s side is incomplete. The mother’s genetics determine whether she personally hyperovulates, but she may have gotten those genes from her father.
Age Also Raises the Odds
Genetics isn’t the only factor. Women over 35 have a naturally higher chance of fraternal twins because FSH levels rise with age. Normally, FSH just barely crosses the threshold needed to develop one dominant follicle each cycle. As women age, FSH levels climb higher and stay elevated longer, which can push multiple follicles past that threshold at the same time. This is the same basic mechanism as the genetic form of hyperovulation, just driven by hormonal changes instead of inherited gene variants.
Research published in The Journal of Clinical Endocrinology & Metabolism found that mothers of hereditary fraternal twins had higher and more pulsatile FSH levels, confirming the hormonal link between genetics and multiple ovulation. In other words, the genes and the aging process work through the same pathway. A woman over 35 who also carries hyperovulation genes has a compounded increase in her chances.
How Common Twins Are by Population
In the United States, the twin birth rate is about 31 per 1,000 live births according to 2023 CDC data. But fraternal twin rates vary dramatically around the world, which further points to a genetic component.
The highest fraternal twinning rates are found in African populations. The Yoruba people of Nigeria have an overall twinning rate of 45 per 1,000 births, with 93% of those being fraternal. This may partly reflect genetics and partly diet: the Yoruba consume a type of yam that contains a compound similar to estrogen, which could stimulate hyperovulation. African Americans also have higher fraternal twinning rates than white Americans.
The lowest fraternal twin rates are found in Asian populations, at roughly 4 per 1,000 births. These population-level differences are too large and consistent to be explained by chance alone, and they persist even after accounting for differences in maternal age and fertility treatment use.
What “Runs in Families” Actually Means in Numbers
Having fraternal twins in your family does increase your odds, but the effect is more subtle than many people assume. Studies analyzing sister-to-sister recurrence rates estimate that genetics accounts for about 26 to 28% of the variation in who has fraternal twins. That means the majority of the variation comes from non-genetic factors like age, body size, and whether fertility treatments were used.
So if your mother or sister had fraternal twins, your chances are higher than average, but far from guaranteed. You’ve inherited a tendency, not a certainty. And if the twins in your family are identical, that tells you nothing about your own odds, since identical twinning doesn’t appear to be hereditary.