Cryopreservation and frozen embryo transfer (FET)
Surplus embryos of day 3 top quality embryos or good-morphology Day 5 or 6 blastocysts in the long agonist group and all the viable embryos/blastocysts in the PPOS group were cryopreserved using vitrification. Those who did not get pregnant in the stimulated IVF cycle and those who postponed embryo transfer would undergo frozen embryo transfer (FET) at least 2 months after the stimulated cycle if they had at least one frozen embryo.
Vitrification was performed with MediCult Vitrifification Cooling (Origio, Denmark) using ethylene glycol, propylene glycol, sucrose as cryoprotectant. Embryos were vitrified one by one at room temperature. For the warming procedure following vitrification, the straw was cut and the capillary was pulled from the straw out of the liquid nitrogen, and immediately warmed one by one using MediCult Vitrification Warming (Origio, Denmark). After warming, embryos were transferred to a culture dish for evaluation and further embryo development. Only embryos with more than 50% of blastomeres present after thawing were transferred in FET cycles.
FETs were carried out in natural cycles for ovulatory women and in clomiphene induced or hormonal cycles for anovulatory women. Up to two embryos or blastocysts were transferred in FET cycles.