Outcome measures
MISCARRIAGE, EARLY PREGNANCY, PREGNANCY LOSS
The independent variable of interest was the duration of progesterone exposure, defined as the number of days from the initiation of progesterone treatment to the completion of embryo transfer. To evaluate whether the duration of progesterone administration before FET affected the clinical outcomes, the primary outcome of the study was clinical pregnancy, which was defined as the presence of one or more gestational sac(s) with an embryonic pole indicating the fetal heartbeat on transvaginal ultrasound at 7 weeks of gestation. Secondary outcomes included biochemical pregnancy, rate of implantation, live births, early pregnancy loss, and miscarriage.Fourteen days after embryo transfer, having a serumβ-hCG level of >5 IU/L was considered a biochemical pregnancy. The implantation rate was determined by dividing the number of intrauterine sacs inspected using transvaginal ultrasound divided by the number of embryos transferred. Live birth was defined as the delivery of viable infant(s) at ≥24 gestational weeks. Early pregnancy loss was considered when no gestational sac was observed even after a serum β-hCG ≥ 5 mIU/mL or loss occurring after the presence of an intrauterine gestational sac was confirmed.