About 1 in every 42 babies born worldwide is a twin, which translates to roughly 12 twin births per 1,000 deliveries. In the United States, the rate is higher: about 31 twin births per 1,000, based on 2023 birth data from the CDC. That baseline number shifts significantly depending on your age, body type, family history, and whether you’re using fertility treatments.
Identical vs. Fraternal: Two Different Odds
Twin pregnancies happen in two fundamentally different ways, and the probability of each follows different rules. Identical twins form when a single fertilized egg splits into two embryos. This occurs in 3 to 4 out of every 1,000 births worldwide, and that rate holds remarkably steady across populations. It doesn’t appear to be influenced by genetics, age, or lifestyle. It’s essentially random.
Fraternal twins are about twice as common. They happen when the ovaries release two eggs in the same cycle and both get fertilized. Unlike identical twinning, fraternal twinning varies widely based on genetics, ethnicity, maternal age, body composition, and fertility treatment. When people talk about twins “running in families,” they’re talking about fraternal twins.
How Maternal Age Changes the Odds
Age is one of the strongest natural predictors of twin pregnancy. Women under 30 have a twin birth rate of about 26 per 1,000 deliveries. For women aged 30 to 39, that rises to roughly 36 per 1,000. Women 40 and older reach about 39 per 1,000, though this number has been declining in recent years as fertility treatment practices have shifted.
The biological reason is straightforward. As women age, their bodies produce more follicle-stimulating hormone (FSH), the hormone that tells the ovaries to prepare eggs for release. Higher FSH levels make it more likely that two eggs will mature and release in a single cycle rather than one. This is why the increase with age applies specifically to fraternal twins, not identical ones.
Family History and Genetics
If fraternal twins run on the mother’s side of the family, her chances of conceiving twins are higher than average. The genetic link is through the mother because the relevant trait is hyperovulation, the tendency to release more than one egg per cycle. A woman who is herself a fraternal twin, or whose mother or sister had fraternal twins, is more likely to carry genes that promote multiple egg release.
The father’s family history doesn’t directly affect the odds. A man can carry genes for hyperovulation and pass them to his daughters, but those genes won’t influence whether his partner releases multiple eggs. So “twins skipping a generation” isn’t quite a myth, but it’s not the whole story either. It’s really about which parent’s side the hyperovulation genes come from and whether they land in a daughter who can express them.
Height, Weight, and Body Composition
Taller and heavier women have a modestly higher chance of fraternal twins. A Norwegian study found that women with a BMI of 30 or above were about 43% more likely to have fraternal twins compared to women at a normal weight. Women 5 feet 8 inches (173 cm) or taller had roughly 28% higher odds, while women shorter than 5 feet 5 inches had about 22% lower odds.
The likely mechanism involves estrogen. Higher body fat increases estrogen production, which can influence how the ovaries respond during each cycle and raise the chance of releasing multiple eggs. These aren’t dramatic increases in absolute terms, since the baseline probability is small to begin with, but they’re consistent enough to show up across large populations.
Fertility Treatments and Twin Probability
Fertility treatments are the single biggest factor that can push twin odds well above the natural baseline. The effect varies enormously depending on the type of treatment.
Ovulation-stimulating medications increase the number of eggs released per cycle. Clomiphene, one of the most commonly prescribed, leads to twin pregnancies in about 7.5% of the pregnancies it produces. Letrozole, an increasingly popular alternative, has a lower twin rate of about 4.7%. Triplet rates also differ significantly: 1.3% with clomiphene versus just 0.2% with letrozole.
IVF carries the highest twin probability when more than one embryo is transferred. Transferring two embryos in women under 35 results in a multiple birth rate around 40%. Even in women aged 38 to 40, double embryo transfer still produces multiples about 28% of the time. By contrast, transferring a single embryo drops the multiple birth rate to somewhere between 0% and 3%, with no significant reduction in the overall chance of having a baby. This is why most fertility clinics now favor single embryo transfer as the standard approach.
Why Twin Rates Have Been Shifting
Global twinning rates hit an all-time high in recent years, with 1.6 million twins delivered annually and the worldwide rate climbing by a third over roughly two decades, from 9 to 12 per 1,000 births. Two forces drove that increase: more women having children later in life and wider use of fertility treatments.
More recently, the trend has started to level off or even reverse in some countries, including the United States. The U.S. twin rate among women 40 and older dropped from about 45 per 1,000 in 2019 to 39 per 1,000 in 2021, an 11% decline in just one year followed by another 4% drop the next. The main reason is the shift toward single embryo transfer in IVF, which dramatically cuts multiple pregnancies without sacrificing success rates. Among younger women, twin rates have held relatively steady.
Putting Your Personal Odds in Perspective
For a woman under 30 with no family history of twins, no fertility treatment, and an average body type, the probability of twins sits around 2.6%, or roughly 1 in 38 pregnancies. If you’re over 35, that edges up to about 1 in 28. Add a strong maternal family history of fraternal twins and the odds climb further, though no one can put a precise number on how much genetics alone contributes.
Fertility treatment is the only factor that pushes twin probability into genuinely high territory. With double embryo IVF in younger women, twins become almost as likely as not. With ovulation medications, the odds are more modest but still several times the natural rate. If you’re not using any fertility assistance, twins remain a low-probability event, just one that’s more common than most people assume.