Transferring one embryo into women during in vitro fertilization (IVF) doesn't lower their chances of giving birth but it does mean fewer women give birth to twins, with all the attendant health risks, a study said.
IVF has traditionally involved transferring multiple embryos, which is known to increase by many times a woman's chances of having twins and other multiple births, and raise risks to both mother and babies, including diabetes during pregnancy, premature birth and cerebral palsy.
But improved technology has helped increase births if single embryos are used in younger women judged to have a good chance of getting pregnant, said Jessica Kresowik at the University of Iowa in Iowa City, who led the study.
"In the past, you did need to implant multiple embryos in order to maintain those pregnancy rates," Kresowik told Reuters Health of the study, published in the journal Fertility and Sterility.
She added that before technology improved, doctors might use six embryos at once, hoping that one would result in a pregnancy. According to the American Society for Reproductive Medicine, no more than two embryos should be transferred for women under 35.
Babies conceived through IVF account for just one percent of births each year, but IVF is responsible for 17 percent of twins, Kresowik added.
Using a single embryo for IVF for all women under 38 who were getting their first round of IVF and had other signs predicting a good outcome became policy at Kresowik's fertility clinic in 2004.
Data for the five years before and after that time showed that birth rates didn't suffer -- and actually improved.
Before the single embryo policy, 51 percent of all women younger than 38 got pregnant and gave birth to a live baby, compared with 56 percent afterwards.
The proportion of women with multiple births dropped from 35 percent to less than 18 percent.
Among 364 women who fit the criteria and had a single-embryo transfer from 2004 on, 65 percent gave birth to a live baby and just over 3 percent had twins or other multiple births.
"A policy like this is a great idea," said Judy Stern, a fertility specialist at the Dartmouth-Hitchcock Medical Center in New Hampshire, who wasn't involved in the study.
"You really can move toward single-embryo transfer and have it give you great success rates, and really low multiple rates," she told Reuters Health.