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.