Modified natural cycles were applied for patients with regular ovulatory cycles, in which ultrasound monitoring was initiated on days 10–12 of the menstrual cycle. When the dominant follicle reached a mean diameter of ≥ 17 mm and endometrial thickness attained ≥ 7 mm, with estradiol > 150 pg/ml and P < 1 ng/ml, the timing of hCG triggering depended on the occurrence of an LH surge. On detection of a serum LH surge (LH ≥ 20 IU/l and more than double the average LH level over the past 2 days), a bolus of urinary hCG (5000 IU; Lizhu Pharmaceutical Trading Co.) was injected in the same afternoon. Exogenous progesterone (400 mg/day; Utrogestan; Besins Healthcare) was given vaginally which started 2 days after hCG administration and day 3 embryo transfer was scheduled 4 days later (6 days later for blastocyst transfer). In the absence of a LH surge (LH < 20 IU/l), hCG was injected at 9:00 p.m. and embryo transfer was arranged 5 days later for 3-day-old embryos or 7 days later for blastocysts. Luteal support was initiated 3 days after hCG triggering.
In patients with irregular menses, endometrial preparation was performed in either a mildly letrozole-stimulated cycle or an artificial cycle (AC), depending on patients’ preference and the discretion of treating physicians. In letrozole-FET cycles, letrozole (Hengrui Medicine Co, Jiangsu, China) was prescribed orally for 5 days initiating on cycle day 3 of spontaneous menses or progesterone-induced withdrawal bleeding, at a daily dose of 5 mg. Ultrasound monitoring and serum hormone analysis were performed from cycle day 10 onwards. If the leading follicle reached a diameter of ≥ 14 mm on cycle day 10, transvaginal ultrasound was repeated every 2 days and no other drugs were added until ovulation triggering. In case of a dominant follicle < 14 mm on day 10, a daily dosage of 75 IU of hMG (Anhui Fengyuan Pharmaceutical Co.) was supplemented to stimulate follicle growth, with incremental doses of 37.5 IU if needed. The timing of ovulation triggering, FET scheduling, and luteal support was the same as above described in natural-FET cycles. In AC-FET cycles, oral 17β-estradiol (Fematon 2 mg, three times daily; Abbott Healthcare Products B.V.) was commenced on the second or third day of a natural or progesterone-induced menstrual cycle. When the endometrial thickness attained ≥ 7 mm, progesterone exposure was initiated. Embryo transfer was performed 3 days after progesterone administration for day 3 embryos or 5 days later for blastocysts. In all study groups, luteal support was continued to 10 weeks of gestation if a pregnancy occurred.
In patients with irregular menses, endometrial preparation was performed in either a mildly letrozole-stimulated cycle or an artificial cycle (AC), depending on patients’ preference and the discretion of treating physicians. In letrozole-FET cycles, letrozole (Hengrui Medicine Co, Jiangsu, China) was prescribed orally for 5 days initiating on cycle day 3 of spontaneous menses or progesterone-induced withdrawal bleeding, at a daily dose of 5 mg. Ultrasound monitoring and serum hormone analysis were performed from cycle day 10 onwards. If the leading follicle reached a diameter of ≥ 14 mm on cycle day 10, transvaginal ultrasound was repeated every 2 days and no other drugs were added until ovulation triggering. In case of a dominant follicle < 14 mm on day 10, a daily dosage of 75 IU of hMG (Anhui Fengyuan Pharmaceutical Co.) was supplemented to stimulate follicle growth, with incremental doses of 37.5 IU if needed. The timing of ovulation triggering, FET scheduling, and luteal support was the same as above described in natural-FET cycles. In AC-FET cycles, oral 17β-estradiol (Fematon 2 mg, three times daily; Abbott Healthcare Products B.V.) was commenced on the second or third day of a natural or progesterone-induced menstrual cycle. When the endometrial thickness attained ≥ 7 mm, progesterone exposure was initiated. Embryo transfer was performed 3 days after progesterone administration for day 3 embryos or 5 days later for blastocysts. In all study groups, luteal support was continued to 10 weeks of gestation if a pregnancy occurred.
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