Abstract: Objective: To explore the relationship between frozen embryo transfer （FET） clinical outcomes and serum level of progesterone on the sixth day in those ＦＥＴ ｃycles combined with hormone replacement. Methods: Those 261 FET cycles combined with hormone replacement were retrospectively analyzed. Those cycles were divided into three groups by serum level of progesterone （P） on the sixth day after FET. Serum level of progesterone was ＜20 μg/L in the group A, 20 μg/L≤P＜30 μg/L in the group B, and ≥30 μg/L in the group C. Clinical features, the pregnancy rate and abortion rate were analyzed. After that, those 261 cycles were divided into three groups by clinical outcomes of FET. The group D had intrauterine fetus, the group E had bad pregnancy outcomes, and the group F had not pregnancy. Then the serum levels of progesterone and estrogen on the sixth day after FET were compared. Results: The pregnancy rate of the group C （５０％） ｗas significantly higher than that of the group B （２８．５％）, and the pregnancy rate of the group B was significantly higher than that of the group Ａ （１1.1%） （Ｐ＜０．０５）. The abortion rate of the group A （６０％） ｗａs significantly higher than that of the group Ｃ （１８％） （Ｐ＜０．０５）． The serum levels of progesterone on the sixth day after FET in the group D and the group E were significantly higher than that of the group F （Ｐ＜０．０５）． Ｃonclusions: The serum level of progesterone on the sixth day after FET combined with hormone replacement can be reasonably used as a marker to predict the clinical outcomes of FET. The higher level of progesterone could mean the better pregnancy outcomes, and the lower abortion rate.

Key words: Hormone replacement therapy, Embryo transfer, Cryopreservation, Progestins, Estrogens