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.
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.
The National Vital Statistics Report also looked at other trends in the U.S. birth rate. Overall, the average age of a woman having her first child in the U.S. rose to 26.3 years old in 2014, a record high, up from 26 in 2013.
It found the number of teenage girls giving birth dropped to a record low in 2014, continuing a steady decline from their peak in 1990. Births to teen moms aged 15-19 were down 0 percent in 2014 compared to the year before. The teen birth rate declined across all racial groups.
Although the report did not analyze the reasons why, there have been increased efforts in recent years to expand the use of long-lasting contraceptives like the IUD or hormonal implants for teens who are sexually active. A program that provided them free for teens in Colorado helped that state lower its teen birth rate 40 percent since 2009. Last year the American Academy of Pediatrics, the nation's most influential pediatricians' group, recommended those birth control methods for sexually active teens.
The report also found that the rate of premature births (less than 37 weeks) dropped slightly from 2013 and was down 8 percent from 2007.
The cesarean delivery rate declined for the second straight year, to 32.2 percent of all U.S. births in 2014. That's still more than most other Western countries, and much higher than the "ideal rate" of 10 to 15 percent of births recommended by the World Health Organization.