NEW YORK >> The twin birth rate hit at an all-time high last year in the United States while the rate for triplets, quadruplets, and higher was the lowest in two decades.

Of the nearly 4 million babies born last year, more than 135,000 were twins. That is about 1 in every 29 babies. In 1980, only 1 in every 53 babies was a twin.

The figures come from the final 2014 birth statistics, which were released Wednesday.

Why the twin increase? More women are waiting until they are older to have babies, and mothers in their 30s are more likely to have twins than younger women.

Experts also point to fertility drugs and procedures like in vitro fertilization, which generally raise the chances of multiple births.

While twin births have been rising for many years, the rate of triplet and higher-order births has fallen 40 percent from its 1998 peak.

That's because doctors have been implanting fewer embryos during in vitro fertilization than in the past, in recognition that more embryos increase the risk of dangerous complications, some experts say.

"What might have been a triplet birth in the past is now a twin birth," said one of the authors of the new report, Michelle Osterman of the Centers for Disease Control and Prevention.

About 1 in every 881 babies born last year was a triplet, quadruplet, or part of a higher number set.

Guidelines urging use of fewer embryos were strengthened following the 2009 "Octomom" case, in which a California woman had octuplets after her doctor transferred 12 embryos.