Cryopreservation and frozen-thawed embryo transfer (FET)
All the day 3 top quality embryos and good-morphology Day 5 or 6 blastocysts were cryopreserved using a vitrification protocol. Women in both groups would undergo frozen–thawed embryo transfer (FET) at least 2 months after the stimulated cycle if they had at least one frozen embryo. FETs were carried out in natural cycles for ovulatory women and in clomiphene induced or hormone replacement cycles for anovulatory women. 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. Up to two embryos or blastocysts were transferred in each FET cycle.
Luteal phase support was given by vaginal or intramuscular progesterone at the discretion of the attending physicians. A urine pregnancy test was carried out 2 weeks after the transfer. Those with a positive urine pregnancy test were scanned after 2 weeks to identify the number and presence of a gestation sac with a fetal pole. All pregnant women were contacted or traced for the pregnancy outcomes after delivery or miscarriage.