About 31 out of every 1,000 births in the United States result in twins, according to 2023 CDC data. That’s roughly a 3% chance in any given pregnancy. You can’t guarantee twins, but several biological factors genuinely shift the odds, and understanding them gives you a realistic picture of what’s within your influence and what isn’t.
How Twins Happen in the First Place
There are two fundamentally different ways twins form. Fraternal (dizygotic) twins happen when your ovaries release two eggs in a single cycle and both get fertilized by different sperm. These twins are genetically no more similar than any two siblings. Identical (monozygotic) twins happen when a single fertilized embryo splits into two. This split can occur at different stages of early development, and it appears to be essentially random. No known lifestyle factor reliably increases the chance of an embryo splitting.
Because identical twinning is unpredictable, nearly every factor that “increases your chances of twins” is really increasing your chances of releasing more than one egg per cycle, a process called hyperovulation. Fraternal twins are about twice as common as identical twins, and they’re the type most influenced by genetics, age, and body composition.
Factors That Increase Your Odds
Family History
Fraternal twins run in families, specifically on the mother’s side. If you have a close female relative (a sister, for example) who had fraternal twins, your own chances are roughly double the general population rate. Researchers have tried to pin down the specific genes responsible for hyperovulation, but results have been mixed. The inherited tendency is real, though, even if the precise genetic mechanism isn’t fully mapped. Your partner’s family history of twins doesn’t directly affect your odds, since twinning depends on whether your body releases multiple eggs.
Maternal Age
Women over 35 are more likely to conceive fraternal twins, and the reason is hormonal. As the ovaries age and egg reserve declines, the pituitary gland compensates by producing more follicle-stimulating hormone (FSH). This hormonal “overshoot” often stimulates multiple follicles to mature in the same cycle. In one study, women who developed more than one large follicle had significantly higher baseline FSH levels (averaging 10.3 IU/l versus 7.7 IU/l) and were older on average (36 versus 34.6 years). So the same biological process that makes conception slightly harder after 35 paradoxically makes twins more likely when conception does occur.
Body Size
Higher body weight is associated with higher twin rates. A large cohort study of over 524,000 births found that twin birth rates increased with increasing pre-pregnancy BMI, with the highest rates in women with a BMI of 40 or greater. Taller women also have a slightly elevated chance. The likely explanation involves insulin-like growth factor, which is higher in larger women and may stimulate the ovaries to release more than one egg. This doesn’t mean gaining weight is a sensible strategy for conceiving twins, but it helps explain why the association exists.
Conceiving While Breastfeeding
One of the more surprising findings: women who conceive while still breastfeeding have a substantially higher rate of twinning. One study found an 11.4% twinning rate among breastfeeding women who became pregnant, compared to 1.1% in the general population. The hormonal environment during lactation, particularly shifts in FSH and other reproductive hormones as cycles resume, likely promotes hyperovulation in some women.
Having Had Previous Pregnancies
Higher parity (having already carried pregnancies to term) is an independent risk factor for twinning. Women who have had several children are statistically more likely to conceive twins in later pregnancies, even after controlling for age.
Fertility Treatments and Twins
The single biggest factor driving twin pregnancies in recent decades has been fertility treatment. Ovulation-stimulating medications work by encouraging the ovaries to mature and release multiple eggs, which directly raises the chance of fraternal twins.
Clomiphene citrate, one of the most commonly prescribed fertility drugs, produces a twin pregnancy rate of about 5.1%, compared to 1.4% in unexposed pregnancies. That’s nearly a fourfold increase. The rate of triplets or higher-order multiples also rises, though it remains rare at about 0.13%.
IVF historically carried even higher twin rates when clinics transferred two or more embryos at a time. The shift toward single embryo transfer has reduced fraternal twins from IVF dramatically, but it hasn’t eliminated twinning entirely. Identical twins occur after single embryo transfer at a rate of about 2.24%, which is two to twelve times higher than the natural identical twinning rate of 0.4%. Embryos transferred at the blastocyst stage (day 5 or 6) split at a higher rate (2.5%) than those transferred earlier (1.7%), suggesting something about the lab culture or transfer process itself makes splitting more likely.
It’s worth being direct: fertility treatments exist to help people who are struggling to conceive, not to engineer twins. No reputable clinic will prescribe ovulation drugs solely to increase the chance of a multiple pregnancy, because the medical risks are significant.
Why Twin Pregnancies Carry Higher Risk
The desire for twins is understandable, but the clinical reality of carrying two babies is substantially different from a singleton pregnancy. In a large multi-center study comparing outcomes, the numbers were striking:
- Preterm labor affected 53.8% of multiple pregnancies versus 5.3% of singletons. More than half of twin pregnancies resulted in early delivery.
- Hypertensive disorders (including preeclampsia) occurred in 19% of multiple pregnancies versus 4.3% of singletons.
- Gestational diabetes was modestly higher at 23.7% versus 19.6%.
- Anemia and postpartum hemorrhage were also significantly more common.
Preterm birth is the most consequential of these risks. Babies born early face higher rates of breathing problems, feeding difficulties, and longer hospital stays. The average twin pregnancy delivers around 36 weeks rather than the typical 39 to 40 weeks for singletons, and many deliver considerably earlier than that.
What You Can Actually Control
If you’re hoping for twins, here’s the honest summary. You can’t choose to have identical twins. No food, supplement, or sexual position has been shown in rigorous research to reliably increase twinning rates. The factors with the strongest evidence, including age, genetics, and body size, are largely things you either already have or don’t.
Conceiving after age 35, conceiving while breastfeeding a previous child, and having a family history of fraternal twins on your side are the natural factors most consistently linked to higher odds. Even combined, they shift your probability from roughly 3% to perhaps 5 to 10%, not to anything close to a guarantee.
Fertility treatments are the only intervention that dramatically raises the chance of twins, and they come with medical oversight, cost, and real health trade-offs for both the mother and babies. If twins happen naturally, that’s wonderful. But pursuing a twin pregnancy as a goal, particularly through medication, means accepting a meaningfully higher-risk pregnancy. Understanding those trade-offs is the most important part of this conversation.