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Health & Pregnancy

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U.S. Twin Birth Rate at All-Time High

Fertility Treatments and Older Age of Pregnant Women Are Top Contributing Factors
By Rita Rubin
WebMD Health News
Reviewed by Louise Chang, MD

Jan. 4, 2012 -- More pregnant women are getting more than they may have expected, according to a report on 30-year trends in twin births.

In 2009, 1 in every 30 babies born in the United States was a twin, compared with 1 in every 53 babies in 1980, according to the “Data Brief” from the CDC’s National Center for Health Statistics.

That represents a 76% jump in the twin birth rate.

Of note though, more than half of twins born from 1980 to 2009 were low birth weight, or less than 5 1/2 pounds, according to the report. One in 10 was very low birth weight, or less than 3 pounds, 5 ounces. Only 1 in 100 single babies is very low birth weight, notes researcher Joyce Martin, MPH.

When Martin first started studying twin birth rates years ago, she says, the impact on the mom’s and babies’ health tended to get overlooked. Instead, Martin says, “The idea was, ‘Isn’t this cute?’” Most twins do well, she says, but they are more likely than single babies to experience complications related to prematurity and low birth weight, such as breathing problems.

Martin and her co-authors attribute the increase in the twin birth rate mainly to the expanding use of fertility treatments, namely ovulation stimulation medications and in-vitro fertilization.

The other major factor is the older age of mothers. In women who’ve conceived without the help of fertility treatments, the chance of having twins increases with age and peaks in the late 30s.

In 1980, about 20% of all births (singles and multiples) were to women aged 30 and older. In 2000-2009, they accounted for more than 35% of all births.

Twin Birth Rates Highest in Women 40 and Older

Because fertility treatments are more commonly used in older women, twin birth rates have been highest among women in their 40s since 1997, the authors write. In 2009, 7% of babies born to women 40 and older came in pairs, compared with 5% of births to women 35-39, and 2% of births to women under 25.

The tide may be turning, though. The pace of the increase in the overall twin birth rate, which on average rose 2% a year from 1980 until 2004, did slow to less than 1% annually from 2005-2009, the authors write.

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