Endometrial preparation
Natural cycle and artificial cycle were used for frozen-thawed blastocyst transfer (FET). Natural cycle was conducted either by administration of HCG for planning the transfer or by detecting the spontaneous LH peak, and blastocysts were transferred on the fifth day after ovulation. In terms of the artificial cycle, 3.75 mg per day of oral oestradiol valerate was started on days 2-3 of the menstrual cycle, and the dose of estradiol valerate was adjusted according to the endometrial thickness monitored by ultrasound. When the endometrial thickness reached ≥ 7 mm, 40 mg/d progesterone was injected 5 days prior to frozen blastocyst transfer. If pregnancy was achieved after frozen blastocyst transfer, luteal phase support was continued until 11 weeks’ gestation.