Determining Factors Influencing The Successful Embryo Transfer and Pregnancy during The Frozen Cycle of In Vitro Fertilization: A Retrospective Cohort Study.

Background: Frozen embryo transfer (FET) has been increasingly used due to advancements in cryopreservation techniques and the safety advantages. This study aims to determine various factors influencing the successful FET.

Materials and methods: Retrospective cohort analysis included 1112 women who underwent programmed FET between January 2012 and October 2022, at King Chulalongkorn Memorial Hospital, Thailand. Patient characteristics, embryo characteristics, endometrial preparation protocol, endometrial characteristics (thickness, pattern), embryo transfer procedure (tip and flow during transfer, embryo placement location, the difficulty of the procedure, presence of blood and mucous at catheter), and operator factor were analyzed. Multiple logistic regression analysis was used to assess the relationship between collected variables and successful embryo transfer which is defined by clinical pregnancy.

Results: The overall clinical pregnancy rate was 34.2%. Women aged 35-40 years and >40 years were less likely to have a clinical pregnancy compared to those aged <35 years [adjusted odds ratio (aOR): 0.523; 95% confidence intervals (CI): 0.360-0.757, P<0.001 and aOR: 0.260; 95% CI: 0.152-0.434, P<0.001, respectively]. Obese women with body mass index (BMI) ≥25 kg/m2 were significantly associated with decreased clinical pregnancy (aOR: 0.632; 95% CI: 0.403-0.978, P=0.042) compared to those with normal BMI. Day-3 and day-4 embryo transfer showed a significant decrease in clinical pregnancy compared to blastocyst transfer (aOR: 0.294; 95% CI: 0.173-0.485, P<0.001 and aOR: 0.497; 95% CI: 0.265-0.900, P=0.024). Double embryo transfer (DET) was 1.78 times more likely to have a clinical pregnancy than women with single embryo transfer (SET) (aOR: 1.779; 95% CI: 1.293-2.458, P<0.001). The cycles with endometrial thickness <8 mm were associated with a decrease in clinical pregnancy compared with those with a thickness ≥8 mm (aOR: 0.443; 95% CI: 0.225-0.823, P=0.013).

Conclusion: Older age, obesity, non-blastocyst transfer, single embryo transfer, and endometrial thickness of <8 mm were significantly associated with a decreased clinical pregnancy in programmed FET.