Advances in obstetrical ultrasound, combined with ancillary magnetic resonance imaging and rapid molecular testing of amniotic fluid, have greatly improved the diagnostic capabilities when assessing the fetus with suspected intrauterine growth restriction (IUGR). Placental villi are covered by the fetally derived epithelial layer termed villous trophoblast; this is a distinct lineage under separate transcriptional control from the extravillous trophoblast (EVT) in mammalian placentae. The maternal-fetal interface, or decidua, is rich in cell lineages of the maternal immune system, in particular large granular lymphocytes and uterine natural killer (NK) cells. The net effect of abnormal maternal perfusion of the placental villi is one or more of the following: spiral artery thrombosis and unstable perfusion. The developmentally abnormal placenta, characterized by decidual bed pathology, structurally abnormal villi with defective or damaged areas of syncytiotrophoblast is prone to thrombosis.