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.