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

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.