The precise molecular and cellular mechanisms that regulate maternal vascular development during gestation are largely unknown. Endothelial progenitor cells (EPCs), which play an important role in vascular homeostasis, have been discovered in the circulation. We examined the level of circulating EPCs throughout uncomplicated pregnancies (n = 20) and assessed the correlation between serum estradiol levels and the number of EPCs. The number of circulating EPCs increased gradually and paralleled the progression of gestational age. In addition, the number of EPCs correlated significantly with the level of serum estradiol. The present study suggests that EPCs may play an important role in the regulation and maintenance of the placental development and vascular integrity during pregnancy.