The success of pregnancy depends on the ability of trophoblast cells to infiltrate the maternal decidua and breach uterine vessels. To ask whether the antigenic phenotype of maternal endothelial cells (EC) in the vascular zone and central decidua basalis may reflect a specialized programming of these vessels for interaction with the trophoblast, we did a survey of several mouse EC differentiation antigens, including MECA-32, MECA-99, and endoglin. Our results revealed striking differences in the phenotype of endothelial lining of vessels in the distinct compartments of the pregnant uterus during Day 9 of pregnancy and at midgestation. Vessels in the central decidua basalis and the vascular zone showed strong expression of MECA-99 but only weak expression of MECA-32, contrasting with the MECA-99(lo), MECA-32(hi) vessels in the capsularis. The vascular zone in addition stained brightly with anti-endoglin. Importantly, invading trophoblast as well as trophoblast cells lining maternal blood spaces were MECA-99(+), MECA-32(-), and endoglin(-), suggesting that the expression of MECA-99 may reflect a specialized co-programming of these trophoblast and EC for future interaction, but also that trophoblast cells may mimic selected antigenic characteristics of endothelium in association with their role in lining maternal blood spaces. In the term pregnant uterus the expression of all differentiation antigens decreased dramatically, suggesting that trophoblast cells as well as maternal EC lose their selected antigenic characteristics when the process of placentation is complete.