Triplets are out, singletons are in and twins are holding steady.
So says a new study in the New England Journal of Medicine that examined birth trends in the United States from1971 to 2011.
The authors of the report, from the Centers for Disease Control and Prevention and other institutions, attribute the shift to professional guidelines urging fertility doctors to transfer only one or two embryos to their patients hoping to get pregnant. Those guidelines were first issued by the Society for Assisted Reproductive Technology in 1998, the same year that rates of triplet, quadruplets and even higher-order multiple births peaked in the U.S.
Overall, the proportion of actual births that involved more than one baby rose from 1.8% in 1971 to 3.5% in 2011. Older women are more likely to give birth to multiples, and when maternal age was taken into account the growth rates were slightly smaller, the researchers found.
Multiple births are a serious medical concern because they involve greater risks for women and their children, the study authors wrote. There are also significant financial consequences both for families and society. (The McCaughey septuplets of Iowa just turned 16, and they told a news reporter they're eager to drive! )
In the past, doctors and patients would opt to transfer three or more embryos during in vitro fertilization procedures to increase their odds of having at least one live birth. But as advances in IVF have boosted success rates, it's gotten easier for fertility specialists to persuade patients to transfer just one or two embryos.
Physicians who perform IVF are required to report data to the CDC's National ART [Assisted Reproductive Technology] Surveillance System. The authors of the new report used that data to calculate that fertility doctors transferred three or more embryos in 79% of IVF procedures in 1998; by 2011, that figure dropped to 24%. As a result, the rate of triplet births due to IVF treatment plunged 79% during that period.
In their place, doctors performed more procedures involving the transfer of fewer embryos. In 1998, only 6% of cases involved a single embryo and 16% involved two. By 2011, those figures were 21% and 55%, respectively, according to the study.
Put another way, 48% of triplet and higher-order births in 1998 were due to IVF, along with 10% of twin births. By 2011, the proportion of higher-order births resulting from IVF fell to 32%, while twin births attributable to IVF rose to 17%.
The same trends were not seen for other types of fertility treatment – including ovulation induction and intrauterine insemination – that are less precise, the study found. In 1998, use of such methods accounted for 36% of triplets and higher-order births, and that figure rose to 45% in 2011. For twin births, the figures were 16% and 19%, respectively.
But the study authors were optimistic that this trend could be turned around as well.
"Increased awareness of multiple births resulting from non-IVF fertility treatments may lead to improved medical practice patterns and a decrease in the rate of multiple births," they wrote.