Clinical factors that affect the pregnancy rate in frozen-thawed embryo transfer in the freeze-all policy.

Background: This study was conducted to analyze clinical factors that can affect pregnancy rates in normal responders undergoing the freeze-all policy in in vitro fertilization.

Methods: We evaluated 153 embryo transfer cycles in 89 infertile women with normal response to controlled ovarian stimulation (COS). After COS, all embryos were cultured to the blastocyst stage, and good quality blastocysts were vitrified for elective frozen-thawed embryo transfer (FET). Clinical variables associated with COS and the results of COS and culture, including the number of retrieved oocytes, fertilized oocytes, and frozen blastocysts were compared between the pregnant group and the non-pregnant group.

Results: After a single cycle of COS for each patient, 52 patients became pregnant while 37 did not. Significant differences were observed in the number of matured oocytes, fertilized oocytes, frozen blastocysts, and transferred embryos. The number of frozen blastocysts in the pregnant group was almost twice that in the non-pregnant group (5.6±3.1 vs. 2.8±1.9, p<0.001). The area under the receiver operating characteristic curve for the 4 frozen blastocysts was 0.801 in the pregnant group.

Conclusion: In the freeze-all policy, the number of matured oocytes, number of fertilized oocytes, and number of frozen blastocysts might be predictive factors for pregnancy.