Multiplying Multiples

Learn why there has been an increase in multiple birth pregnancies in recent years and what to expect if you're having more than one.

Multiple births seem to dominate the news and celebrity magazines. That's not surprising. During the past 25 years or so, the number of twin births in the United States has increased 65 percent, and the number of higher-order multiples (triplets or more) has increased by more than 400 percent, according to the National Center for Health Statistics. Approximately three of every 100 babies born in the United States are multiples.

Why the increase? Part of the reason is that women in the United States are waiting longer to have babies. As a woman ages, her chance of having multiples goes up. Assisted reproduction techniques (ART) such as in vitro fertilization also contribute to the rise in multiples. About half of the births that result from ART are multiples.

Your likelihood of multiples. Multiples are more likely to be born to mothers who have them in their family. According to recent studies, maternal height and weight come into play too. Women in the top 25th percentile of height and those with a body mass index of 30 or greater (which is considered obese) are more likely than thinner, shorter women to have multiples.

Twins are most common. The vast majority of multiple births are twins; they account for about 95 percent of multiple births. Twins can be identical or fraternal. Identical twins, which account for one-third of twin sets, occur when one fertilized egg splits early in pregnancy and develops into two fetuses. With fraternal twins, each fetus develops from a separate egg and sperm. Identical twins share one placenta and amniotic sac; fraternal twins each have their own placenta and amniotic sac.

The chance of having identical twins versus fraternal twins is not linked to age, family history, or anything else that scientists have uncovered so far. Having identical twins is believed to be an unpredictable, random event.

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