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 pregnant or planning to become pregnant, here are some factors that may increase the odds you’ll find yourself eating for three. 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. In women with a BMI over 30 (the clinical definition of obesity), the incidence of fraternal (but not identical) twins increased by 30% to 60%. 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. However, it’s not a good idea to gain weight to increase your odds of having twins. There is no guarantee, and having a higher BMI increases your risk of miscarriage and gestational diabetes. Despite being a flawed measure, BMI is widely used today in the medical community because it is an inexpensive and quick method for analyzing potential health status and outcomes. 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. Again, this phenomenon appears linked to genetics and applies only to fraternal twins. A rare exception involves tennis star Roger Federer and his wife Mirka, who have two sets of identical twins (pictured). 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. On average, the rate of twinning in this country is around 3% overall. In people who take Clomid, that can increase by anywhere from 6% to 11.7%. 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 transferred; a single transferred embryo can sometimes divide and lead to monozygotic twins. 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. According to the 2007 review, 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. 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.
