Fertility and pregnancy outcomes following fresh versus frozen-thawed embryo transfer in assisted reproductive technology

Abstract

Background and aims: Assisted reproductive technology has provided great hope for millions of infertile couples. In recent years, frozen-thawed embryo transfer has no longer been merely an add-on to the conventional fresh embryo transfer. Transfer of a cryopreserved embryo has contributed to nearly half of the embryo transfer cycles in Australia. The aim of this thesis is to evaluate the fertility and pregnancy outcomes following fresh versus frozen-thawed embryo transfer. Materials and methods: The thesis includes four studies using population-based data extracted from Australian and New Zealand Assisted Reproduction Database and Victorian Assisted Reproductive Treatment Authority. The risks of ectopic pregnancy, small/large for gestational age birth, and cycle-based live birth following frozen-thawed embryo transfer were compared with those of fresh embryo transfer. This thesis also investigated the cumulative live birth rate from one oocyte retrieval following a ‘fresh transfer’ strategy versus a ‘freeze-all’ strategy. Results: Compared with fresh embryo transfer, frozen-thawed embryo transfer was associated with a decreased risk of ectopic pregnancy and small for gestational age birth, but an increased risk of large for gestational age birth. The ‘freeze-all’ strategy resulted in a similar cumulative live birth rate as the ‘fresh transfer’ strategy among high responders (>15 oocytes), but did not benefit normal (10–15 oocytes) and suboptimal responders (<10 oocytes). Vitrification was the preferred cryopreservation method for blastocysts. Conclusion: This thesis provides population-based evidence of fertility and pregnancy outcomes following fresh versus frozen-thawed embryo transfer. This thesis suggests that, from a population perspective, the ‘freeze-all’ strategy may benefit some subgroups of patients, but should not be offered universally.