For the average pregnancy in the United States, the chance of having twins is about 3.1%, or roughly 1 in 33 births. The most recent CDC data (from 2023) puts the twin birth rate at 30.7 per 1,000 live births. Your individual odds, however, can shift significantly based on your age, family history, body composition, ethnicity, and whether you’re using fertility treatments.
Identical vs. Fraternal: Two Different Odds
Fraternal twins, which develop from two separate eggs fertilized by two different sperm, are about twice as common as identical twins. These are the twins whose odds fluctuate based on personal factors, because they depend on a process called hyperovulation, where the ovaries release more than one egg in a single cycle. Anything that increases the likelihood of releasing multiple eggs increases the chance of fraternal twins.
Identical twins form when a single fertilized egg splits into two embryos. This happens at a relatively stable rate of about 3 to 4 per 1,000 births, and it doesn’t appear to be strongly influenced by age, genetics, or ethnicity. It’s essentially a random event. The one exception is IVF, where identical twinning occurs at a slightly higher rate of 1 to 3 per 100 pregnancies, possibly due to the laboratory handling of embryos.
How Age Affects Your Chances
Women over 35 have a higher chance of conceiving fraternal twins than younger women. As you age, your body produces more follicle-stimulating hormone to compensate for declining fertility. That extra hormonal push can cause the ovaries to release two eggs at once. The twin birth rate peaks for women in their late 30s, then drops after 40 as overall fertility declines sharply. So there’s a sweet spot in the mid-to-late 30s where twinning odds are at their highest for natural conception.
Family History and Genetics
Fraternal twins do run in families, but only on the mother’s side and only in a specific way. What’s inherited is the tendency toward hyperovulation. If your mother or maternal grandmother had fraternal twins, your chances are higher than average. The father’s family history of twins doesn’t affect the odds, because twinning depends on the egg-releasing patterns of the person who’s pregnant.
Researchers have looked for specific genes responsible for hyperovulation, but the results have been mixed. No single gene has been definitively identified as the “twin gene” in humans. The trait likely involves multiple genes working together, which makes it hard to predict with precision. If fraternal twins run heavily in your maternal line, you may have roughly double the average odds, but there’s no genetic test that can tell you for certain.
Identical twins, by contrast, don’t follow family patterns. Having identical twins in your family doesn’t raise your chances of having them.
Ethnicity and Twin Rates
Twin birth rates vary meaningfully across ethnic groups. In the U.S., non-Hispanic Black women have the highest twinning rate at about 40.5 per 1,000 births. Non-Hispanic white women come next at around 34.3 per 1,000, and Hispanic women have the lowest rate at approximately 24.4 per 1,000. That means a Black woman in the U.S. is roughly 65% more likely to have twins than a Hispanic woman.
Globally, the pattern holds in a broader sense. Central African countries have some of the highest natural twinning rates in the world, while East Asian countries tend to have the lowest. These differences appear to reflect population-level variation in the genes that influence hyperovulation.
Body Size Plays a Role
Women with a BMI of 30 or higher are nearly 1.5 times more likely to have twins than women with a BMI in the normal range of 20 to 25. The effect is most pronounced for fraternal twins. One possible explanation is that higher body fat increases levels of certain hormones, including insulin-like growth factor, which may stimulate the ovaries to release more than one egg. Taller women also appear to have a slightly elevated chance of twins, though the evidence on height is less robust than for BMI.
Previous Pregnancies Matter
If you’ve already been pregnant before, your odds of having twins go up slightly with each subsequent pregnancy. Data from multiple countries shows that among women of the same age, those with more prior births have a higher probability of a multiple birth. The effect is modest, but it stacks on top of the age factor, since women with several children also tend to be older. A 37-year-old on her fourth pregnancy has meaningfully higher twin odds than a 25-year-old on her first.
Fertility Treatments and Twin Odds
Fertility treatments are the single biggest factor that can raise your chances of twins, and the type of treatment matters enormously.
Ovulation-stimulating medications like clomiphene carry a twin rate of about 7.5% per pregnancy (roughly 1 in 13). Another commonly used medication, letrozole, has a lower twin rate of about 4.7% per pregnancy. Both work by encouraging the ovaries to produce multiple eggs, but letrozole appears to do so with less risk of multiples. Triplet rates also differ significantly: 1.3% with clomiphene versus just 0.2% with letrozole.
IVF twin rates depend almost entirely on how many embryos are transferred. Most clinics now transfer a single embryo in over 90% of cycles, which has dramatically reduced twin births from IVF. With a single embryo transfer, the chance of twins drops to just 1 to 3%, and those rare twins are identical (from the embryo splitting) rather than fraternal. When two embryos are transferred, twin rates jump to roughly 30 to 40%, which is why the medical field has moved decisively toward single transfers.
Putting It All Together
Your baseline chance of twins is about 3%, but your personal odds land somewhere on a wide spectrum. A younger woman of average weight with no family history of twins and no fertility treatment is looking at closer to 1 to 2%. A woman over 35 with a maternal family history of twins, a higher BMI, and prior pregnancies could be at 4 to 5% or more without any medical intervention. Add fertility medication, and the odds jump to 5 to 8%. Transfer two embryos during IVF, and the chance rises above 30%.
The U.S. twin rate actually peaked around 2014 at about 33.9 per 1,000 births and has since declined to 30.7, largely because fertility clinics shifted to single embryo transfers. So while more people are using assisted reproduction than ever, the twin rate is slowly coming down as medical practices change.