Live birth after a freeze-only strategy versus fresh embryo transfer in three randomized trials considering progesterone concentration

Research question: Is there a difference in live birth rate between a freeze-only strategy and fresh embryo transfer, and what is the effect of varying progesterone concentrations on the day of human chorionic gonadotrophin (HCG) administration? Design: A secondary analysis of data from three randomized trials comparing the live birth rate after elective frozen versus fresh embryo transfer, which respectively enrolled 1508 women with polycystic ovary syndrome, 2157 ovulatory women who underwent cleavage-stage embryo transfer and 1650 ovulatory women who underwent single blastocyst transfer. Women were randomly assigned to the frozen or fresh embryo transfer group in the original trials. The primary outcome was live birth rate after the initial embryo transfer. Results: The live birth rate after a freeze-only strategy was consistently higher than fresh embryo transfer at any progesterone concentration on the day of HCG administration. Nonetheless, the between-group difference in live birth rate after frozen versus fresh embryo transfer was greater in women with progesterone concentrations ≥1.14 ng/ml (52.7% versus 37.3%, odds ratio (OR) 1.88, 95% confidence interval (CI) 1.55–2.27, P = 7.89 × 10^–11) than in women with progesterone concentrations <1.14 ng/ml (53.3% versus 48.1%, OR 1.23, 95% CI 1.08–1.41, P = 0.002). In women with progesterone concentration ≥1.14 ng/ml, frozen embryo transfer also resulted in higher rates of conception and clinical pregnancy than fresh embryo transfer. Conclusion: In women with normal or high ovarian response, a freeze-only strategy resulted in a higher live birth rate than fresh embryo transfer, irrespective of progesterone concentration. Moreover, women with progesterone concentration ≥1.14 ng/ml may benefit more from a freeze-only strategy.