Spiral artery remodeling is essential step of establishing uteroplacental circulation in human pregnancy. In early pregnancy, extravillous trophoblasts invade into maternal spiral artery, forming endovascular trophoblast plugs, and these endovascular trophoblasts undergo a phenotypic switch to express CD56, a NK cell marker. Little effort is made to understand what happen to these endovascular trophoblasts after initial implantation. They die through apoptosis or necrosis ? How these endovascular trophoblasts relate to placental growth and development, especially placental lateral growth? These endovascular trophoblasts are within the maternal circulation, and measuring these endovascular trophoblasts during pregnancy will serve as a biomarker for pregnancy complications related to maternal circulation such as preeclampsia.