<p>Background: Low serum progesterone &nbsp;concentration on the day of frozen embryo transfer (FET) has been associated with adverse pregnancy outcomes. Progesterone supplementation has been shown to improve the outcomes in these cycles. The purpose of the current study was to investigate the prevalence, pregnancy outcomes, and factors associated with rescued FET cycles involving low serum progesterone concentrations.<br /> Methods: A cross-sectional study was conducted on 367 FET cycles with hormonal endometrium preparation (oral estradiol+vaginal progesterone) at Hung Vuong Hospital, Vietnam, from October 2022 to February 2023. Serum progesterone concentrations were measured on the day of FET. All cycles with serum progesterone &lt;10 <em>ng/ml </em>were supplemented with intramuscular progesterone according to the hospital protocol, and outcomes were subsequently observed.<br /> Results: The prevalence of cycles with low serum progesterone concentration was 71.66% (263/367). Factors associated with low serum progesterone were female body weight (ORadj=1.04; 95%CI: 1.0006&ndash;1.07) and duration from the last progesterone dose to blood sampling (ORadj=1.11; 95%CI: 1.03&ndash;1.19). Despite being rescued with progesterone supplementation, cycles with serum progesterone &lt;10 <em>ng/ml</em> had significantly lower chemical (ORadj=0.52; 95%CI: 0.31&ndash;0.89), clinical (ORadj=0.54; 95%CI: 0.31&ndash;0.93) and ongoing (ORadj=0.54; 95%CI: 0.31&ndash;0.94) pregnancy rates.<br /> Conclusion: A high prevalence of frozen-thawed cycles with low serum progesterone concentrations was observed in this study, which was associated with female body weight and duration from the last progesterone &nbsp;dose to blood sampling. Despite progesterone rescue, lower pregnancy rates were detected in cycles with serum progesterone &lt;10 <em>ng/ml</em>.</p>