Researchers have said that fewer triplets, quadruplets and higher multiple sets of babies are being born from in-vitro fertilization in the United States due to more restrictive guidelines. No longer do doctors routinely implant numerous embryos in a woman who is trying to become pregnant, said the findings in the New England Journal of Medicine.
But new treatments, such as drugs that stimulate the ovaries into producing more eggs, are picking up some of the slack.
The percentage of twin births in the United States nearly doubled over the past 40 years, going from 1.8 percent in 1971 to its peak of 3.5 percent in 2009, the study found.
Triplet and higher multiple births reached a high point in 1998 and then dropped off sharply, declining almost 30 percent from 1998 to 2011, said the study.
US guidelines changed in 1998 to discourage implantation of three or more embryos at a time.
After that, the percentage of triplets or more born due to IVF fell from 48 percent of cases in 1998 to 32 percent in 2011.
Meanwhile, other non-IVF procedures have accounted for an increasing share of multiple births, rising from 36 percent to 45 percent from 1998 to 2011, the study said.
Increasingly popular procedures like ovulation induction and ovarian stimulation can be unpredictable, making it more difficult to prevent multiple births, the study said.
Non-IVF procedures accounted for slightly more twin and higher order multiple births than traditional IVF in recent years.
Given the health risks to mothers and babies, senior author Eli Adashi said the United States is grappling with "a real problem" of "way too many multiple births."
"IVF is moving, in a sense, in the right direction and cleaning up its act, whereas the non-IVF technologies are at a minimum holding their own and possibly getting worse," said Adashi, professor of obstetrics and gynecology at Brown University.
The first successful attempt at IVF -- defined as procedures in which eggs and sperm are manipulated outside the body to establish a pregnancy -- was in 1977.
The world's first so-called "test-tube baby," Louise Brown, was born in 1978. About five million have since joined the ranks of IVF-conceived babies.
The treatment became mainstream in the 1990s, even though the high cost of IVF, up to $15,000 per cycle, has kept the procedure out of reach for many.
All kinds of medically-assisted conception are behind a significant portion of multiple births in the United States today -- by 2011, 36 percent of twin births and 77 percent of triplets and higher, said the study.
The rise in multiple births in the United States is attributable both to women having babies at older ages and the increase in available medical treatments for infertile couples.
The study showed that women over 30 are particularly likely to undergo fertility treatments that result in twin and multiple births.