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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 2010, 61,564 infants were born using an a form of assisted reproduction technology (ART), and 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...