RESUMEN
OBJECTIVE: This study set out to investigate the pregnancy outcome of natural cycle regimen versus other endometrial preparation protocols with vitrification thawed blastocyst transfer (VTBT) cycles. METHODS: This control trial study was carried out on 123 women undergoing VTBT. The women were randomly divided into three groups of endometrial preparation before VTBT; 1. Modified natural ovulation cycle with using HCG (n=32) 2. Mild hormonally stimulated cycle by low dose Clomiphene Citrate (n=30) and 3. Artificial cycle induced with estradiol and progesterone supplementation (n=61). Following endometrial preparation, the thawed blastocyst was vitrified and transferred. Reproductive outcome and endometrium characteristic were evaluated in the three groups. RESULTS: The three above-mentioned protocols resulted in clinical pregnancy rates of 21.43% vs. 13.79% vs. 15.25%, respectively; without statistical differences. The ongoing pregnancy rates did not show any significant differences among the three groups (21.43% vs. 13.79% vs. 13.56%), respectively. In addition, the miscarriage rates were compared in the three groups. The endometrial thickness on the day of progesterone or human chorionic gonadotropin administration were more frequently observed in the artificial and modified natural cycle versus hormonally stimulated groups (8.34±0.89 vs. 7.3±1.4, p<0.001; 8.13±0.95 vs. 7.3±1.4, p<0.001). There was no significant difference regarding triple-line endometrial patterns in the three groups. CONCLUSION: The natural cycle with HCG trigger could be considered as an alternative protocol to mild hormonally or artificial cycle regimens in vitrification thawed blastocyst transfers.