For the first time in nearly 40 years, the twin birthrate in the U.S. is on the decline. According to a data brief published Thursday from the National Center for Health Statistics, twin births declined in the U.S. by 4% from 2014 to 2018. The decline follows decades of steady growth which began in the 1980s and lasted through the early 2000s.
The leading theory for the trend reversal is that fertility treatments — generally thought to have caused the twin boom in the first place — have gotten more sophisticated. "Changes to fertility enhancement therapies are definitely part of the equation," says Joyce Martin, statistician with the National Center for Health Statistics, who co-authored the brief.
In vitro fertilization has improved, says Dr. Kevin Doody, director of the Center for Assisted Reproduction Facility in the Dallas-Fort Worth area of Texas and a past president of the Society for Assisted Reproductive Technology. Fertility specialists used to transfer multiple embryos at a time, he says, which improved the odds of having a successful birth but increased the likelihood of twins.
He says now doctors can achieve the same success rates with lower chance of multiple gestation.
"The technology is much more mature," says Doody, who was not an author of the brief. "We went from being able to grow embryos in the laboratory for two days to now it's five or six days, and that enables us to better select healthy embryos." He adds that improvements to specimen-freezing technology have also played a role in keeping success rates consistent while reducing the incidence of multiple gestation.
The rise in the rate of twins in the U.S. coincided with the introduction and wide adoption of fertility treatments, like in vitro fertilization, and drugs that induce ovulation. When the first baby in the U.S. was conceived with assisted reproductive technology in 1981, twins accounted for roughly one out of every 53 births. By 2014, twins had become nearly twice as common, at one out of every 29 births.
Doody points out that IVF is only a piece of the puzzle, and says he thinks that fertility specialists have also changed their approach for non-IVF fertility treatments that carry high risk of multiple pregnancy. "As we became more aware of the health risks of multiple pregnancy and made efforts to reduce it with IVF, I think that's translated to a reduction in doing things that are risky from an ovulation induction or super-ovulation standpoint."
The NCHS brief notes that the downward statistical trend is only observed among non-Hispanic white mothers. Twin birthrates remained fairly steady among Hispanic and black mothers during the same time period (2014-2018) that white mothers experienced a 7% decline. Doody say this finding reinforces his speculation that changes to fertility treatments are driving the trend shift.
"Access to fertility treatments, unfortunately, has some socioeconomic basis," Doody says. "So this trend isn't going to reflect as much in groups that aren't accessing fertility treatments as frequently, and that might be the non-Hispanic black and Hispanic population."
The decline was also much more pronounced in older mothers, with mothers over 40 experiencing a 23% decrease in twin births from 2014 to 2018. The twin birthrate for mothers aged 29 and younger remained relatively stable. Again, Martin and Doody attribute this to changes in fertility treatments. "Older mothers are more likely to receive these treatments," says Martin.
The trend is notable from a public health perspective. "Babies born in twin pregnancies are more likely to be born too early and too small and are more likely to die in the first year of life [than] singletons," Martin says. And speaking of triplets and multiples, Martin says that those birthrates are also on the decline, but because "triplets and multiples are at much higher risk of poor outcome than twins, we try to talk about them separately whenever possible."
"I've written a lot of special reports on twins and triplets over the years," says Martin. "But when you see a decline like this, it's definitely noteworthy."
Noteworthy, and — to some — welcome. "I'm very pleased to see the reduction," says Doody. "When it comes to twins — it's not 'buy one, get one free.' From a medical standpoint, it's a risk."