Objective: To evaluate whether progesterone (P4) supplementation in patients with low serum progesterone levels after normal luteal phase support (LPS) improves pregnancy outcomes in women undergoing frozen embryo transfer (FET). Methods: A retrospective, single-center cohort study including 1301 vitrified-warmed frozen embryo transfer (FET) cycles conducted from May 2021 to July 2023 at Vinmec Times City International Hospital. A total of 696 FET cycles from individual patients were analyzed for pregnancy outcome. Patients with serum P4 level ≥ 10 ng/ml (Normal P4, n=359) continued to receive standard LPS, while those with P4 levels < 10 ng/ml (Rescue P4, n=337) received 50 mg intramuscular P4 injection daily in addition to the standard LPS for 7–10 days. Results: Daily intramuscular P4 effectively restored serum P4 levels to that of the Normal P4 group and resulted in pregnancy outcome comparable to that of patients with P4 levels ≥10 ng/ml. Notably, the efficacy of this protocol was consistent even in very low serum P4 concentrations (< 4ng/ml) and was independent of preimplantation genetic testing for aneuploidy (PGT-A). Conclusion: Intramuscular P4 effectively increased serum P4 levels resulting in pregnancy and neonatal outcomes comparable to those in patients with P4 levels ≥10 ng/ml. This approach provides a safe, effective intervention to improve FET outcomes, supporting the routine inclusion of serum P4 testing and intramuscular P4 rescue in standard FET protocols.
