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Drugs that help women become pregnant have replaced in vitro fertilization as the main culprit behind high-risk multiple births, according to a study looking at births of triplets and higher-order multiples.

"IVF, which is usually the one we tend to point fingers at, was not the leading culprit," says Eli Adashi, a professor of obstetrics and gynecology at Brown University who was senior author of the study, published Wednesday in the New England Journal of Medicine.

The number of multiple births of triplets or more due to fertility drugs like clomiphene citrate and injectable hormones rose from 36 percent to 45 percent from 1998 to 2011. At the same time, the births of multiples because of IVF dropped from 48 percent to 32 percent.

Having more than one baby at a time increases the risk for both mother and children, including premature birth, cerebral palsy and developmental delays. Between 1971 and 2011, the proportion of multiple births doubled in the United States, the study finds, from 1.8 percent to 3.5 percent. Almost all of that was due to...