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Background: The aim of this study was to investigate whether, in a large randomized trial, it is possible to identify specific maternal and/or embryo variables that could independently correlate with ongoing implantation in IVF/ICSI. Methods: In a Scandinavian study, 661 women were randomized to elective single embryo transfer or double embryo transfer. Women aged <36 years undergoing their first or second IVF cycle and with at least two good quality embryos were eligible. Only one cycle per subject was included. In the present study, cycles with 0 or 100% ongoing implantation (n = 520) were analysed regarding maternal and embryo variables. Results: In this selected study group, the ongoing implantation rate was 195/734 (26.6%). In the univariate analysis, first IVF cycle, conventional IVF as fertilization method and 4-cell embryos showed a statistically higher ongoing implantation rate than did second IVF cycle, ICSI and non-4-cell embryos. In the multivariate analysis the same variables correlated independently to ongoing implantation. In addition, ovarian sensitivity correlated independently to ongoing implantation. Conclusion: This information should be used when selecting the number of embryos for transfer with the overall aim to reduce the rate of multiple births while maintaining a satisfactory birth rate. © The Author 2005. Published by Oxford University Press on behalf of the European Society of Human Reproduction and Embryology. All rights reserved.

BACKGROUND: The risks of premature birth and perinatal death are increased after in vitro fertilization. These risks are mainly due to the high incidence of multiple births, which relates to the number of embryos transferred. METHODS: We performed a randomized, multicenter trial to assess the equivalence of two approaches to in vitro fertilization with respect to the rates of pregnancy that result in at least one live birth and to compare associated rates of multiple gestation. Women less than 36 years of age who had at least two good-quality embryos were randomly assigned either to undergo transfer of a single fresh embryo and, if there was no live birth, subsequent transfer of a single frozen-and-thawed embryo, or to undergo a single transfer of two fresh embryos. Equivalence was defined as a difference of no more than 10 percentage points in the rates of pregnancy resulting in at least one live birth. RESULTS: Pregnancy resulting in at least one live birth occurred in 142 of 331 women (42.9 percent) in the double-embryo-transfer group as compared with 128 of 330 women (38.8 percent) in the single-embryo-transfer group (difference, 4.1 percentage points, 95 percent confidence interval, -3.4 to 11.6 percentage points), rates of multiple births were 33.1 percentand 0.8 percent, respectively (P<0.001). These results do not demonstrate equivalence of the two approaches in rates of live births, but they do indicate that any reduction in the rate of live births with the transfer of single embryos is unlikely to exceed 11.6 percentage points. CONCLUSIONS: In women under 36 years of age, transferring one fresh embryo and then, if needed, one frozen-and-thawed embryo dramatically reduces the rate of multiple births while achieving a rate of live births that is not substantially lower than the rate that is achievable with a double-embryo transfer. Copyright © 2004 Massachusetts Medical Society.