REDUCING THE RISK OF MULTIPLE BIRTHS BY TRANSFER OF 2 EMBRYOS AFTER IN-VITRO FERTILIZATION

Background In vitro fertilization is associated with a high risk of mu ltiple births, which is a direct consequence of the number of embryos transferred. However, other factors that contribute to the risk are no t well defined. Methods Using the data base established by the Human F ertilisation and Embryology Authority in the United Kingdom, we studie d the factors associated with an increased risk of multiple births in 44,236 cycles in 25,240 women. The factors included the woman's age, t he cause and duration of infertility, previous attempts at in vitro fe rtilization, previous live births, number of eggs fertilized, and numb er of embryos transferred. Results Older age, tubal infertility, longe r duration of infertility, and a higher number of previous attempts at in vitro fertilization were all associated with a significantly decre ased chance of a birth and of multiple births. Previous live birth was associated with an increased chance of a birth but not of multiple bi rths. The higher the number of eggs fertilized, the higher the likelih ood of a live birth. When more than four eggs were fertilized, there w as no increase in the birth rate for women receiving three transferred embryos as compared with those receiving two, but there was a conside rable increase in the rate of multiple births when three were transfer red (odds ratio, 1.6; 95 percent confidence interval, 1.5 to 1.8). Con clusions Among women undergoing in vitro fertilization, the chances of a live birth are related to the number of eggs fertilized, presumably because of the greater selection of embryos for transfer. When more t han four eggs are fertilized and available for transfer, the woman's c hance of a birth is not diminished by transferring only two embryos. T ransferring more embryos increases the risk of multiple births. (N Eng l J Med 1998;339:573-7.) (C) 1998, Massachusetts Medical Society.