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

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):≦2 embryos 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.