Twins Are Becoming Less Common in U.S., for Good Reasons

THURSDAY, OCT. 3, 2019 (HealthDay News) -- No, you're not seeing double as often these days: After decades of rising, twin births are declining in the United States.

Twin birth rates had been on the rise for 30 years, but dropped 4% between 2014 and 2018, health officials said in a new U.S. government study. That's the lowest level in more than a decade. In 2018, there were 32.6 twins for every 1,000 U.S. births.

So what's going on? Experts suspect the decline probably stems from improved techniques for assisted reproduction.

"We know from other sources that there have been improvements in fertility-enhancing therapies, in particular in reproductive technologies," said Joyce Martin, an epidemiologist for the National Center for Health Statistics (NCHS), part of the U.S. Centers for Disease Control and Prevention.

With these new methods, fewer women are having more than one embryo implanted, she explained. It used to be that several embryos were implanted, leading to the surge in twins and triplets.

"So you're seeing the decline among older moms, who are more likely to have these therapies, and among white moms, who are also more likely to have these therapies," Martin said. She's the lead author of the study published Oct. 3 in the CDC's NCHS Data Brief.

Between 2014 and 2018, the number of twins born in the United States dropped about 2% a year, the study found. Nearly 124,000 twins were born last year.

Though twin birth rates fell by 10% or more for mothers starting at age 30, the decline was greatest among women 40 and older, and it was only seen in white women, the researchers found. Twin birth rates for black and Hispanic women were unchanged.

Despite these declines, the birth rate for twins is still way above what it was in 1980, when 1 in every 53 births was a twin.

Having twins can be problematic, Martin said. Many are born preterm, so they weigh less.

"Twins are seven times more likely to be born too early and three times more likely to die within the first year of life," she said.

Martin predicts the twins' birth rate will continue to decline as assisted reproductive technologies improve.

For the study, her team used data from the U.S. National Vital Statistics System.

It revealed that twin births nationwide peaked in 2007 at nearly 139,000.

Between 2014 and 2018, the data showed significant declines in twin birth rates in 17 states and significant rises in three: Arizona, Oklahoma and Idaho.

In 2018, twin birth rates ranged from 24.9 per 1,000 in New Mexico to 36.4 in Michigan and Connecticut. Forty-five states and the District of Columbia had twin birth rates of 30 per 1,000 (3%) or more.

Dr. Rahul Gupta, chief medical health officer at the March of Dimes, agreed that better reproductive technology explains the trends.

"Fewer embryos transferred result in fewer multiple births," he said. "I hope that one of the reasons is that we are getting to the point where a single embryo is transferred."

Gupta said the decline in multiple births among older women is a positive development. He noted that women in their 30s and 40s are more likely to develop complications during pregnancy such as high blood pressure, preeclampsia and gestational diabetes.

These problems, along with chronic health conditions such as obesity, can increase the risk to both mother and baby, Gupta said. To reduce the chances for a bad outcome, he recommends women should be in their best physical shape before they get pregnant.

Gupta advised women who are considering assisted reproduction to ask their doctor about single embryo implantation and other updated technology to improve their odds for a healthy outcome.

More information

For more on assisted reproduction, see the U.S. Centers for Disease Control and Prevention.