The process of in vitro fertilization — in which embryos are created outside a woman’s body and then implanted in her uterus — has become increasingly common in the United States. In 2014, an estimated 70,352 infants were born using an a form of assisted reproduction technology (ART). Almost all of those resulted from IVF.
A common IVF practice has involved transferring multiple embryos to a woman’s body in one cycle. This was thought to increase the likelihood that at least one embryo would successfully result in a live birth. The average number of embryos transferred at one time is two to three.
While this strategy makes sense theoretically, it is not risk free. Transferring more than one embryo creates a risk for multiple pregnancies, such as twins or triplets, which makes the pregnancy higher risk and increases the risk of premature birth and low birth weight. (Single pregnancies created via IVF are also thought to be at a higher risk for prematurity, low birth weight, and congenital anomalies, though researchers aren’t certain whether this is due to the IVF techniques or the underlying infertility problem.)
The research has been convincing. In a joint practice committee statement, the Society for Assisted Reproductive Technology and the American Society for Reproductive Medicine concludes that with improving technology, single embryo transfer (SET) is an increasingly better choice for achieving pregnancy while avoiding multiples.
The statement notes that SET may be particularly appropriate for women with the best chance of a good outcome, such as those who are under 35 or on their first or second treatment cycle, and that women ages 35 to 40 could also elect SET if they have “top quality” embryos of the proper stage available for transfer.
The authors also note that the United States has “lagged behind” the rest of the world in focusing on SET. For example, the National Institute for Health and Care Excellence (NICE) in the UK recommends single embryo transfer for most women and no more than two at a time for anyone.