But new research finds that implanting just one embryo into a woman seeking to become pregnant still significantly increases the odds of twins.
Artificial reproduction techniques, such as stimulating the ovaries or implanting several embryos in a woman's womb at once, are well known to raise the chances of twins, triplets and more. Doctors have tried to minimize the risk of multiple pregnancies -- which are associated with prematurity and other health problems for the children -- by implanting only one embryo at a time into a woman's uterus.
However, a study in the June 5 issue of the New England Journal of Medicine shows that identical, or monozygotic, twins occur six times more often after in vitro fertilization with a single embryo than normal -- 2.3 percent of births versus 0.4 percent.
"Even if you opt to do a single embryo transfer, which is currently the best available procedure, the risk is still higher to have [twins] compared to spontaneous conception," says study co-author Dr. Isaac Blickstein, a fertility specialist at Hebrew University in Jerusalem. "You still have a sixfold increased chance that this egg will split."
The reason for the greater chances isn't clear, Blickstein says. One unproved theory is that during assisted reproduction, the outer wall of the manipulated egg becomes slightly perforated. As it develops, it begins to split and ultimately winds up as two separate embryos.
Blickstein and his colleagues in the United States and Britain report their findings as a research letter in the journal.
The rate of twins, triplets, quadruplets and other multiple births among U.S. women has risen more than fourfold since 1980, according to the U.S. Centers for Disease Control and Prevention. The increase has been most dramatic for women in the 35-to-39 age bracket, for whom the rate of such deliveries rose nearly tenfold between 1980 and 1997.
In previous work, Blickstein had estimated the risk of identical twins associated with in vitro fertilization might be 12 times the normal chances. But that study was based on a smaller group of patients.
This time, he and his colleagues reviewed birth records from British in vitro fertilization clinics between 1991 and 1998. During that time, 1,104 babies were born through single-embryo transfer procedures. Of those, 44 were twins and nine were triplets.
Whether the embryos were freshly created or had been frozen didn't affect their chances of splitting into twins, the researchers say. Nor did the odds vary depending on whether the eggs had been put back quickly following fertilization or after a delay.
Dr. Louis Keith, a Chicago obstetrician and a co-author of the study, says the findings have "enormous negative clinical implications." Identical twins are vulnerable to several potentially serious birth complications, from prematurity to conjoinment, says Keith, of Northwestern University's Feinberg School of Medicine.
Dr. Robert Brzyski, president of the Society for Assisted Reproductive Technology, says couples should be made aware of the greater chances of having identical twins when they seek fertility treatments.
"We don't understand all the mechanisms, and there are some small risks associated with high-technology conception," says Brzyski, of the University of Texas Health Science Center in San Antonio. "Those need to be considered in evaluating the risks and benefits."