7 Factors That May Increase the Chance of Having Twins

One may be hiding on your Thanksgiving dinner table

For some parents, the idea of having twins is exciting. For others, it's scary. Either way, once a double bundle of joy arrives, new moms and dads will be too happy and busy (and exhausted) to look back on how they felt before their babies were born.

Twins occur when one fertilized egg separates into two embryos, creating monozygotic, or identical twins, or when two eggs are fertilized by different sperm, which results in dizygotic, or fraternal twins. While there is no real way to know ahead of time if either of these events will take place during conception, if you're a woman who's pregnant or planning to become pregnant, here are some factors that may increase the odds you'll find yourself eating for three.

Your Genes

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Genetics play a big role in determining whether you'll have twins or not. Research suggests that a woman's chance of having twins is double if her mother or sister had twins.

But, interestingly enough, this only applies to fraternal twins. Among families with a history of twins, there are few with identical twins.

What this suggests is that genetics somehow play a role in multiple ovulation (also known as hyperovulation), in which more than one egg is released during a menstrual cycle.

Your Height or Weight

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A study published in the journal Fertility and Sterility found that mothers with a high BMI (body mass index) were significantly more likely to have fraternal twins than women of normal weight.

In women with a BMI over 30 (the clinical definition of obesity), the incidence of fraternal (but not identical) twins increased by 30 percent to 60 percent.  Height also is a factor: Tall women, who fall in the top 25th percentile, are more likely to have twins than their more petite peers.

Of course, this doesn't mean that gaining weight will increase your odds of having twins. What it does increase, however, is your risk of miscarriage and gestational diabetes if your BMI crosses the threshold toward obesity.

Your Age

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Numerous studies have shown that women 35 years and older are more likely to conceive fraternal twins than their younger counterparts. It is thought the genetic changes that occur with aging can accelerate and alter the way in which a woman ovulates.

According to the American College of Obstetricians and Gynecologists, women of "advanced maternal age" are more likely to release more than one egg during ovulation.

It's important to note that being pregnant as an "older" woman poses some risks including miscarriage, gestational diabetes, and chromosomal abnormalities like Down syndrome.

Having Had Twins Already

Roger Federer two sets of twins
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Once you've had one multiple pregnancy, you're significantly more likely to have twins again. Research suggests that mothers of twins are four times more likely to have twins again than mothers with singletons or those never previously pregnant.

Again, this phenomenon appears linked to genetics and applies only to fraternal twins. A rare exception involves tennis star Roger Federer and his wife Micka who had two sets of identical twins (pictured).

A Diet Rich in Yams

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The Yoruba tribe in Nigeria has the highest rate of twin births in the world. Researchers have linked this phenomenon, in part, to a diet rich in cassava (a type of yam). The peel of this vegetable is thought to contain a compound (phytoestrogen) that may promote hyperovulation.

Scientists looking at the incidence of twins among the Yoruba believe genetics also may play a role. It appears that the propensity for twinning remains high among women who remain in the tribe as opposed to those have moved elsewhere and had children with non-Yoruban men.

Infertility Treatment

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Reproductive technologies have dramatically increased the rate of multiple births in the United States. For instance, the fertility drug Clomid (clomiphene) works by stimulating ovulation and sometimes cause the release of multiple eggs in a single cycle (referred to as superovulation).

On average, the rate of twinning in this country is around three percent overall. In women who take Clomid, that number can increase to around six percent, according to researchers at the Yale University School of Medicine.

Multiple births are even more likely to result from in vitro fertilization (IVF). In this case, it is not just that multiple embryos may be implanted; the transferred embryos can sometimes divide and lead to monozygotic twins.

Going Off the Pill

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It has long been suggested that stopping your birth control pills may cause overstimulation of the ovaries and lead to hyperovulation. It is believed that the sudden termination of the Pill may cause a spike in the production of follicle stimulating hormone (FSH) central to ovulation.

When this happens, the body may over-respond and release multiple eggs at once. Most studies suggest that the effect is only temporary and will normalize in a relatively short period of time.

Studies have conflicted over the years, with some reporting statistically significant increases in fraternal twins and others showing no association at all. Still, if you'd prefer not to have twins, use an alternate form of birth control for a few cycles after you stop taking the Pill. 

Pure Luck

Mom with twin babies
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Many parents of multiples don't meet any of the criteria for twinning yet find themselves with two babies without even trying. Monozygotic twins are particularly surprising since no one is quite sure what can cause an egg to split after conception. It remains a mystery.

Want to try to have twins? There's no sure-fire way to improve your odds, and even if there were, keep in mind that the risks and complications associated with a multiple pregnancy can be significant. These include preterm birth, low birth weight, preeclampsia, and miscarriage.


American College of Obstetricians and Gynecologists. “Multiple Pregnancies.” Washington, D.C.; FAQ188, July 2015.

Hoekstra, C.; Willemsen, G.; van Beijsterveldt, C. et al. “Body Composition, Smoking, and Spontaneous Dizygotic Twinning.” Fertil Steril. 2010; 93(3):885-93; DOI 10.1016/j.fertnstert.2008.10.012.

Hoekstra, C.; Zhao, Z.; Lambalk, C. et al. “Dizygotic Twinning.” 2008; 14(1):37-47; DOI 10.1049/humupd/dmm036.

Legro, R.; Brzyski, R.; Diamond, M. et al. “Letrozole Versus Clomiphene for Infertility in the Polycystic Ovary Syndrome.” N Engl J Med. 2014; 371:119; DOI 10.1056/NEJMoa1313517.

Peek, P.  Twins in African and Diaspora Cultures. Bloomington, Indiana: Indiana University Press. 2011.