The twin birthrate has declined in the United States after rising for years, a new report said this week. One theory researchers have put forward to explain the change is that fertility therapies that previously involved transfers of multiple embryos are less common.
The National Center for Health Statistics said in the report that the twin birthrate had declined by an average of 1 percent a year from 2014 through 2018, when the rate was 32.6 per 1,000 births. That was the lowest rate in more than a decade, it said.
The number of twin births more than doubled from 1980 to 2007, when there were 138,961 such births, the peak level, the report said. The number fluctuated until 2014, when there were 135,336. In 2018, there were 123,536 twin deliveries.
“It is trending downward for the first time in three decades,” said the report’s lead author, Joyce A. Martin, a statistician with the N.C.H.S. “What we don’t have is a good explanation of why these rates might be declining. However, the data do suggest one possibility.”
The new report relied on data from birth certificates, which supplied information about race, birth numbers and the states in which the births took place.
The decline in the twin birthrates was detected in older mothers and non-Hispanic white mothers, Ms. Martin said.
“And we know one of the largest factors influencing the rise in previous years was their use of fertility-enhancing therapies,” she said. “They are the most likely to use those therapies.”
“Since the decline was limited to those two groups, it does suggest some changes might be happening,” Ms. Martin said.
Ms. Martin was referring to improvements in reproductive technologies like in vitro fertilization, which involves the stimulation of ovaries to produce several eggs, which are removed and fertilized with sperm before the resulting embryos are transferred into the uterus.
Some I.V.F. experts discourage multiple births. Although doctors in the past commonly transferred two or more embryos in hopes of achieving a pregnancy, advances in the development of the embryos have meant that doctors are increasingly encouraged to transfer just one or two at a time to avoid risky pregnancies and births.
Multiple embryos accounted for a higher incidence of twins, but that practice has tapered off over the years amid advances in embryo quality, improving the chances of implantation and birth.
In the mid-1990s, transfers of single embryos accounted for only about 5 percent of such procedures in the United States, said Dmitry Kissin, the assisted reproductive technology team leader at the Centers for Disease Control and Prevention. Preliminary data for 2017 showed that more than 60 percent of transfers happened with just one embryo.
Advances such as better freezing of extra embryos, genetic testing, and culturing embryos for a longer period have meant that fewer embryos are transferred, Dr. Kissin said. “The chance of implanting is much higher,” he said.
Many of those factors might be driving the overall trend in the birthrate data, Ms. Martin said. “It does suggest those changes might be influencing the declines,” she said.
