Comparison of the Pregnancy Outcomes of Frozen Embryo Transfer in the Natural Cycle vs. Artificial Cycle

This prospective, randomized controlled clinical trial is conducted on women aged 18 to 42 years undergoing frozen embryo transfer (FET) during in vitro fertilization (IVF) cycles. Eligible participants meet the inclusion criteria and provide written informed consent.

Participants are randomized into two groups using a permuted block method:

HRT Group (Control): Estradiol valerate is initiated at 4 mg/day on day 2 of the menstrual cycle and is adjusted to 8 mg/day based on endometrial thickness. Once the endometrial thickness reaches ≥8 mm, progesterone supplementation is started (400 mg vaginal suppositories twice daily and 50 mg intramuscular injections daily) for 5 days before blastocyst transfer.

Natural Cycle Group: A transvaginal ultrasound is performed on cycle day 2 or 3 to rule out ovarian cysts and confirm an endometrial thickness <5 mm. If eligible, patients are monitored from day 8 onward. Ovulation is tracked by assessing dominant follicle growth (≥15 mm), LH surge (≥17 IU/L), and serum progesterone (>1.5 ng/mL). Once ovulation is confirmed and endometrial thickness reaches ≥7 mm, FET is scheduled for 6 days post-LH surge or 5 days post-follicle collapse. Serum progesterone is measured one day before transfer; if <10 ng/mL, 400 mg/day progesterone is administered.

Embryos are cultured in Blastocyst medium (Origio, Denmark) and assessed daily. High-quality embryos (Grade A) are defined by a 4-cell stage on day 2, a 7-8 cell stage on day 3, uniform cell size, and <20% fragmentation. Blastocyst grading follows Gardner's criteria. One or two top-quality embryos are transferred using a Cook catheter under abdominal ultrasound guidance. If no blastocyst is available by day 5, the most advanced embryo is transferred.