A new controversial study suggests a stronger embryo may help a weaker one survive in twin pregnancies created by in vitro fertilization (IVF).
Spanish researchers found that the overall rate of survival to birth per embryo was 83 percent in twin pregnancies compared with 76 percent when women carried only one child.
In a statistical analysis, the number of double births in twin pregnancies was higher than the researchers would have expected if the embryos didn't somehow help each other.
By contrast, the number of single births was much lower than they expected.
As a result, the write in the journal Obstetrics and Gynecology, it would appear that feeble embryos that would fail on their own somehow receive support from their stronger twin.
While several studies have found that twin pregnancies result in higher live birth rates, the concept of "embryo assistance"—still entirely theoretical—met resistance from one expert not involved in the new study.
"The authors have offered an interesting hypothesis," Dr. Alan Copperman, director of reproductive endocrinology at Mount Sinai School of Medicine in New York, told Reuters Health by email.
"But their data are not sufficient to overturn our current understanding that embryo implantation is independent, and we do not have evidence that individual embryos 'help each other' implant."
Babies conceived through IVF account for just one percent of U.S. births each year, researchers say, but the technique is responsible for 17 percent of twins.
Doctors often implant more than one embryo to increase the chances of a successful pregnancy, a practice that's particularly common in the U.S.
But given that multiple pregnancies carry risks for the fetuses, clinics have recently been cutting back on the number of embryos they transfer into women.
The American Society for Reproductive Medicine recommends that no more than two embryos should be implanted in women under 35—and doctors should consider using just one.
Earlier this year, a study from an Iowa fertility center found no drop in successful pregnancies after it introduced a single-embryo policy (see Reuters Health story of October 21, 2011).
The new study is based on data from 1,159 single and 523 twin pregnancies (none of which were identical twins, which result when a fertilized egg splits and develops into two embryos).
Seventy-two percent of the twin pregnancies yielded double live births, while the expected rate was just 58 percent.
The difference in live birth rates between the single and twin pregnancies were particularly pronounced in women over 33, notes Fernando Miro and colleagues from the Hospital Clinic in Barcelona.
According to the team, this "strongly suggests that the contingent embryos present some deficiency."