In a first since the 1980s, the twin birthrate drops in the US

A recent report by the Centers for Disease Control and Prevention shows that the twin birthrate has dropped in the United States after rising for more than 30 years.

Experts say it could have something to do with improvements in assisted reproductive technology.

The twin birth rate declined by 4% between 2014 and 2018, reaching 32.6 twins per 1,000 total births in 2018, the lowest rate in more than a decade. The decline affected mothers over the age of 40, and non-Hispanic white mothers, while the rate of twin births among non-Hispanic black and Hispanic mothers was essentially unchanged.

The number of twin births more than doubled from 1980 to 2007, when it reached a peak of 138,961. After that, the twin birthrate fluctuated. It increased by about 2% a year from 1980 to 2003, slowed to less than 1% annually from 2003 through 2014, and then dropped by 4% between 2014 and 2018.

In 2018, 123,536 twins were born in the US, which is still 1.8 times more than in 1980 (pdf).

Experts have linked the rise in twin births to the increased use of in-vitro fertilization or IVF, which used to involve transferring two or more embryos into a woman's womb. But advances in reproductive technology means doctors now avoid doing this to prevent risky pregnancies and births.

Having twins is riskier for both mother and child. It increases the risk of premature birth, diabetes, preeclampsia, and miscarriage, among other conditions. Women who give birth to twins or multiple babies are also more likely to experience postpartum depression.

The CDC report was based on birth certificates collected by the the National Vital Statistics System, which contains registration data for the United States dating to the 1960s.