Growth differentiation factor-8 (GDF-8) is found in the human serum, follicular fluid and granulosa cells. Our previous studies have shown that the human cumulus expansion and steroidogenesis can be regulated by GDF-8. However, thus far, the expression profile of GDF-8 in serum and whether the level of serum GDF-8 influences pregnancy results for patients treated with in vitro fertilization/intracytoplasmic sperm injection-embryo transfer (IVF/ICSI-ET) is totally unknown. In this study, we showed that GDF-8 had a dynamic trend during controlled ovarian hyperstimulation (COH) procedure. On human chorionic gonadotropin (hCG) administration day, patients with a GDF-8 level higher than 4.7 ng/ml had lower progesterone levels and a higher pregnancy rate. From hCG day to oocyte pick-up day, patients with a GDF-8 decrease greater than 1.3 ng/ml had a higher progesterone increase and a higher pregnancy rate. Importantly, the levels of GDF-8 were negatively correlated with progesterone levels. Our findings provide evidences that GDF-8 plays an important role in ensuring successful pregnancy by regulating progesterone levels.