Endometrial preparation
Natural cycle and artificial cycle were used for FET. Natural cycle was conducted either by administration of hCG for planning the transfer or by detecting the spontaneous LH peak, and cleavage-stage embryos were transferred on the third 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 3 days prior to frozen embryos transfer. If pregnancy was achieved, luteal phase support was continued until 11 weeks’ gestation.