Natural Cycle Frozen Embryo Transfer: Evaluating Optimal Protocols for Preparation and Timing

Area of Science:

• Reproductive endocrinology
• Infertility treatments
• Assisted reproductive technologies

Background:

• Natural cycle frozen embryo transfer (NC-FET) is increasingly utilized.
• Significant practice variations exist in NC-FET protocols, including ovulation induction, trigger administration, and embryo transfer timing.
• There is no established consensus on the optimal NC-FET protocol.

Purpose of the Study:

• To evaluate the association between key NC-FET protocol aspects and cycle outcomes.
• To assess the impact of letrozole use, ovulation trigger method, and transfer timing on implantation, pregnancy, and live birth rates.

Main Methods:

• Retrospective cohort study of blastocyst stage NC-FET cycles (October 2019 - July 2021).
• Evaluated three primary predictors: letrozole use, exogenous trigger vs. spontaneous LH surge, and trigger-based vs. progesterone-based timing.
• Primary outcomes included implantation, clinical pregnancy, and ongoing pregnancy rates.

Main Results:

• 183 cycles from 170 patients were analyzed.
• Average implantation rate was 0.58; clinical pregnancy rate was 59.0%; ongoing pregnancy rate was 51.4%.
• No significant associations were found between any of the evaluated protocol variations and implantation, clinical pregnancy, ongoing pregnancy, or live birth rates after adjustment.

Conclusions:

• Various NC-FET preparation and timing protocols may yield comparable outcomes.
• Flexibility in protocol selection based on patient and physician preference may be possible.
• Findings require validation through larger, randomized trials.