The practice of implanting multiple embryos during in vitro fertilization (IVF) to improve the odds of a successful pregnancy has been a factor—along with the use the use of fertility drugs—in the rising rate of multiple births in the United States. Multiple births increase both the risk and the financial cost of complications during pregnancy and delivery; they also increase the incidence of birth defects. Groups such as the American Society for Reproductive Medicine (ASRM), which represents providers of infertility treatment, note that U.S. patients often choose to implant more embryos than recommended in part because IVF procedures are expensive and generally not covered by insurance (TGR, Vol. 2, No. 5, October 1999).

A study published in November in the Journal of the American Medical Association, however, concludes that for younger women, implanting three or more embryos during IVF, does not improve the odds of a successful birth, although it does significantly increase the risk of multiple births. The study, conducted by Centers for Disease Control and Prevention researchers and including more than 35,500 IVF procedures nationwide in 1996, found that the chances of a live birth peaked at 43% among women aged 20-29 when two embryos that were judged to be of high quality were implanted. The rate of multiple births per live birth among the same age group, however, doubled (from 23% to 46%) when three embryos were implanted rather than two. The study also found that both the live-birth rate and the odds of a multiple birth decreased dramatically with age; they were 24% and 25%, respectively, among women aged 40-44, even when five embryos were implanted.

These findings have prompted ASRM to revise its IVF guidelines to recommend that a maximum of two embryos be transferred for most younger women; ASRM's previous guidelines had recommended a maximum of three embryos for such patients. For older women and for women for whom treatment has failed repeatedly in the past, ASRM recommends transferring up to five embryos.