Demographers credit the boom in twin births since the 1980s to women choosing to become mothers at older ages, using fertility treatments to get pregnant.

The twin birthrate has declined in the United States after rising for three decades. A new report released by the National Center for Health Statistics shows that the twin birthrate fell 4 percent from 2014 to 2018, from 33.9 per 1,000 total births to 32.6 per 1,000 total births after years of increases.

The NCHS paper shows that the number of twin births, which more than doubled from 68,339 in the 1980s to a peak of 138,961 in 2007, declined to 123,536 in 2018.

Starting in 1980, the twin birthrate of 18.9 per 1,000 total births rose an average of 2 percent annually through 2003. The pace then slowed down to less than 1 percent annually through 2014. The recent decline was most sharp in women ages 30 and over. For those ages 30-34, it fell 10 percent. For ages 35-39, 23 percent. And for women ages 40 and over by 23 percent.

The twin birthrate was essentially the same for women in their 20s, according to the report.

One theory researchers put forward to explain the change is that fertility therapies, that previously involved transfers of multiple embryos, are less common.

In the mid-1990s, transfers of single embryos accounted for only about 5 percent of such procedures in the United States. Preliminary data for 2017 showed that more than 60 percent of transfers happened with just one embryo.

The decline in the twin birthrates was detected in older mothers and non-Hispanic white mothers. We know one of the largest factors influencing the rise in previous years was their use of fertility-enhancing therapies. They are the most likely to use those therapies”, said the report’s lead author, Joyce A. Martin, a statistician with the N.C.H.S. “Since the decline was limited to those two groups, it does suggest some changes might be happening,” Ms. Martin said.

In the past, many fertility doctors transferred multiple embryos during in vitro fertilization to increase the chances that at least one would implant successfully. More than a few of those pregnancies resulted in twins or higher-order multiples. But now many procedures transfer only one at a time. Many clinicians also advocate using lower doses of ovulation-stimulating hormones than in the past to avoid possible side effects.

Older women accounted for 20.5 percent of all births in 2014, but for 25.3 percent of all births in 2018. It is among these older women that the twin rates have diminished the most.

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