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BACKGROUND: In vitro fertilization is associated with a high risk of births, which is a direct consequence of the number of embryos. However, other factors that contribute to the risk are not well. METHODS: Using the data base established by the Human Fertilization Embryology Authority in the United Kingdom, we studied the factors with an increased risk of multiple births in 44, 236 cycles in25, 240 women. The factors included the woman's age, the cause and duration of, previous attempts at in vitro fertilization, previous live, number of eggs fertilized, and number of embryos transferred. RESULTS: age, tubal infertility, longer duration of infertility, and a higher of previous attempts at in vitro fertilization were all associated with significantly decreased chance of a birth and of multiple births. Previous birth was associated with an increased chance of a birth but not of births. The higher the number of eggs fertilized, the higher the of a live birth. When more than four eggs were fertilized, there no increase in the birth rate for women receiving three transferred as compared with those receiving two, but there was a considerable in the rate of multiple births when three were transferred (odds, 1. 6; 95 percent confidence interval, 1. 5 to 1. 8). CONCLUSIONS: Among undergoing in vitro fertilization, the chances of a live birth are to the number of eggs fertilized, presumably because of the greater of embryos for transfer. When more than four eggs are fertilized and for transfer, the woman's chance of a birth is not diminished by only two embryos. Transferring more embryos increases the risk of births.
Templeton et al. (Mon,) studied this question.