Twin birth rates hit an all-time high in the United States

By Rachel Feltman-December 28

The CDC has released a final report on births in the United States in 2014, showing an all-time record number of twins. We're not talking about a monumental increase, here - 2014 saw 33.9 sets of twins per 1,000 births, versus 33.7 in 2013 - but several factors suggest this increase could become a trend.

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STAT points out that mothers continued to get older in 2014. The average age for a first-time mother is now 26.3, up from an even 26 in 2013. The teen birth rate is dropping sharply (9 percent in 2014 alone, with some states seeing decreases higher than 50 percent since 2007) due to better efforts to educate teens and provide them with longterm birth control like IUDs. Births for women in the first few years of their twenties are on a slower (but still steady) decline. Meanwhile, the birth rate for women in their 30s, 40s and even 50s have increased over the years to varying degrees.

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Why does this mean more twins? There are actually a couple of factors likely at play here. For starters, even though conceiving is generally more difficult for older moms, these women are actually more likely to have twins, at least according to some research. As women reach the end of their reproductive years, their bodies produce higher quantities of the hormone that triggers the maturation of an egg-producing follicle. That means a woman is more likely to have two eggs available for fertilization during a single cycle in her 30s than she is during her 20s. This doesn't always result in fraternal twins, because older eggs are also more likely to be unviable. But when enough women in this age group get pregnant, a significant portion of them can be expected to have fraternal twins.

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The second factor is that many of these older mothers are using IVF to conceive. In the past, IVF caused a sharp increase in the number of births with three babies or more, because doctors would implant a high number of embryos in the hopes of creating at least one viable pregnancy. As of 2013,more than one-third of U.S. twins and three-quarters of triplets (and higher) births could be linked to IVF. But now triplet (and higher) births are at a 20-year record low, even as twins continue to rise.

This shift is almost certainly due to a 1998 change in guidelines that discouraged doctors from implanting more than two embryos at a time. Now that IVF success rates are so high, implanting just two embryos (or even just one) is often sufficient, and implanting more than that puts mothers at a high risk of carrying a dangerous multiple pregnancy.

For more information on how births have changed in recent years, check out the rest of the CDC's report.

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Rachel Feltman runs The Post's Speaking of Science blog.