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   It's not uncommon, when you're trying to conceive, to wonder whether you might get more babies than you bargained for — especially since multiple births are multiplying at such a fantastic rate. Currently in the U.S., there are 32 pairs of twins for every 1,000 babies born. That's up 65 percent in the past 20 years!

Why such a huge increase in twin births? Much of it's due to advances in assisted reproductive therapy (ART), of course, but there are also other factors at play, at least when it comes to fraternal twins (identical twins are rare and occur at random, so they're even more of an exciting surprise!). Your chances of seeing double (or more) go up if:

You have twins in your family. Heredity plays a role in conceiving twins — but only on the mom's side. Even if your partner's family tree is loaded with dynamic duos, that won't increase your own odds of conceiving twins.You're African-American. Black women are a bit more likely than Caucasians to have twins; if you're Asian or Hispanic, then you're a bit less likely than Caucasians to get two-for-one.You're an older mom-to-be. As you age, your chance of conceiving twins increases. Researchers have found that women over 35 produce more follicle stimulating hormone (FSH) than younger women, which may cause more than one egg to be released in one cycle.You've been pregnant before. With each pregnancy, the odds that you'll have twins go up.You've had twins before. Lightning can and does strike twice! If you've already given birth to a set of twins, look out — you're twice as likely to do so again.You're undergoing fertility treatment. ART, as I mentioned, is a big contributor to the increase in multiple births. About 20 to 25 percent of women taking ovary-stimulating drugs or undergoing in vitro fertilization (IVF) will conceive twins or higher-order multiples.You're overweight. Recent studies have shown that obese women are twice as likely to give birth to fraternal twins as those who aren't obese.You're tall. The same studies found that taller women tend to face higher odds of having fraternal twins.

http://www.whattoexpect.com/preconception/ask-heidi/twins.aspx