The Effect of Additional Embryo Transfer on the Pregnancy Rate in Young Women Receiving in vitro Fertilization: A Natural Experiment Study

Abstract

Purpose: To assess the impact of additional embryo transfer (ET) on pregnancy in young women, we used a natural-experiment approach. Patients and Methods: The design was based on the national policy of South Korea limiting the number of embryos transferred in vitro fertilization (IVF):<= 2embryos on day 2-4 or one on day 5-6 for patients aged <35, with one extra embryo allowed for patients aged >= 35. Using the data from 1909 ET cycles of 1287 women aged >= 34 and <= 35, we calculated adjusted risk ratios (RRs) for pregnancy. Results: Half of cycles were undertaken by women aged 35, and additional ET was performed in 68.7% of them. Intrauterine pregnancy (45.2% vs 51.3%) and multiple gestation (30.5% vs 6.9%) were more common in women aged 35 than in those aged 34. The RR for intrauterine pregnancy was 1.34 (95% confidence interval: 1.12-1.59) when comparing double ET to single ET in frozen day 5-6 cycles. Conclusion: We observed no evidence of a higher probability of pregnancy with additional ET in fresh or frozen day 3-4 ET, or in fresh day 5-6 ET of women aged 35. Additional ET may not increase the successful pregnancy rate in the 35-year-old group, unless it is a frozen day 5-6 ET cycle.