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.