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Twins, triplets, quadruplets, and even bigger broods hold seemingly endless fascination. Identical siblings dressed in identical outfits and stories of twins separated at birth invite questions about nature, nurture, and human individuality. Reality shows like Jon and Kate Plus 8 rake in the ratings.

According to the Centers for Disease Control and Prevention, the odds a baby born in the US will be part of a multiple delivery are 1 in 29.7. With a little more than 4.25 million total recorded births in America in 2006 (the same year from which the birth statistics were derived), that’s about 145,000 multiple births.

As you might expect, multiple births are most likely to be twins, and the rates dwindle as the number of babies rises—from 137,000 twin births (or 274,000 twins born) in 2006 all the way down to 355 quadruplet births and only 67 sets of five or more. The odds a baby will be part of a twin birth are 1 in 31.12; a triplet birth, 1 in 697.2; a quadruplet birth, 1 in 12,020; and a birth of quintuplets or more, 1 in 63,660. The odds of being sent to the emergency department because of a snow blower accident in a year are better, at 1 in 50,320.

But not all women who get pregnant face the same odds. Whenever an “Octomom” hits the front pages, one of the first questions asked is whether fertility treatment played a role. Because of the follicle-stimulating hormones and numerous implanted embryos that may be involved, the chances of having a multiple birth jump if a woman had medical help to conceive. The CDC estimates that so-called assisted reproductive technologies, including in vitro fertilization and intrauterine insemination, contributed to 43.3% of triplets-or-more births in 1997. Fertility medications without additional technology accounted for another 40%, while just 20% of multiple births were natural, or “spontaneous.”

The likelihood a woman will have more than one kid at a time also rises as she ages, because older ovaries are more likely to release more than one egg at ovulation. The combination of more older women conceiving, either naturally or with infertility treatment, contributed to a rise in multiple birth rates over the past several decades.

The odds go up again if a woman has a family history of fraternal twins and multiple births. (A predisposition for fraternal multiples, which happen when ovaries release multiple eggs, can be inherited; identical multiples, which happen when a single fertilized egg spontaneously splits, haven’t been linked to genetics.) Similarly, a woman who’s had one multiple birth is more likely to have another.

Then there are stranger associations, like eating dairy products from hormone-infused cows. But researchers don’t advise going vegan (or quitting the dairy-free life) as a family planning option just yet.