Spotlight on Assisted Reproductive Technology
By Maurizio Macaluso, MD, DrPH
Chief, Women's Health and Fertility Branch
Division of Reproductive Health
Centers for Disease Control and Prevention
Assisted reproductive technology (ART) has been used in the United States since 1981 to help women become pregnant, most commonly through in vitro fertilization (IVF) of human eggs followed by transfer of the embryos into the woman’s uterus. In 1992, Congress passed the Fertility Clinic Success Rate and Certification Act, requiring Centers for Disease Control and Prevention (CDC) to collect data from all ART clinics and report success rates, defined as live births per ovarian stimulation procedures, for each ART clinic. In a typical ART procedure, eggs are retrieved from a woman’s ovary, combined with sperm in the laboratory, and the resulting embryo(s) are transferred back into the woman’s uterus or fallopian tubes. An ART cycle is defined as a process in which (1) a woman has undergone ovarian stimulation or monitoring with the intent of having an ART procedure (even if the cycle was subsequently canceled and no embryos were transferred) or (2) embryos previously frozen have been thawed with the intent of transferring them to a woman. Types of ART procedures include in vitro fertilization (IVF) with and without intracytoplasmic sperm injection (ICSI), gamete intrafallopian transfer (GIFT), and zygote intrafallopian transfer (ZIFT).
Data from the National ART Surveillance System (NASS) indicate that in 2006, 138,198 ART cycles were performed at 426 reporting clinics in the United States, resulting in 41,343 live births (deliveries of one or more living infants) and 54,656 infants. Although use of ART is still relatively rare as compared to the potential demand, it has doubled over the past decade and ART-born infants now account for over 1 percent of all United States-born infants. This proportion is larger in states where statutes mandate insurance coverage of infertility treatment.
A critical decision in the performance of ART is about how many of the embryos obtained through IVF will be transferred back to the uterus with the purpose of establishing a pregnancy. In general, the more embryos are transferred the higher the likelihood that at least one will implant, giving rise to a clinical pregnancy and a live birth. Currently, almost 75 percent of the procedures performed in the United States involve the transfer of two or three embryos. Elective single embryo transfer, however, is not widely practiced. Only about 11 percent of IVF cycles in 2004 involved the transfer of just one embryo. Multiple embryo transfer is associated with an increased probability of establishing a multifetal pregnancy. Whereas ART-born infants account for over 1 percent of all United States-born infants, they represent 18 percent of all multiple births.
Multifetal pregnancies increase the risk for maternal complications such as gestational diabetes. Multiple-birth infants are at increased risk for low birth weight, preterm delivery, infant death and disability among survivors. Recent systematic reviews of the literature indicate that ART-conceived singletons also face increased risks for low birth weight, very low birth weight, preterm delivery, and fetal growth restriction. These findings have been confirmed in population-based studies in the United States. As many of the ART singletons are delivered after a multifetal pregnancy, a proportion of the adverse outcomes observed in these infants are attributable to multiple embryo transfer. A recent analysis used six years of data (2000-2005) from the National ART surveillance system, and national birth certificate data during the same period to compute the proportion of low birth-weight infants born after ART, whose low birth weight status is attributable to the practice of multiple embryo transfer. The results, presented at the 2009 conference of the European Society for Human Reproduction and Embryology in Amsterdam, suggest that between two thirds and three quarters of the burden of low birth weight experienced by ART infants in the United States is due to multiple embryo transfer, and that the underlying infertility condition (a factor also known to be independently associated with adverse birth outcomes) is likely to explain no more than 10 percent. Thus, it is appropriate to expect that widespread adoption of single embryo transfer could translate in a large reduction of the burden of low birth weight and possibly other adverse infant outcomes associated with ART in the United States.