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.”