Presence of Nucleated Red Blood Cells

Nucleated red blood cells (nRBCs) originate in blood islands of the yolk sac at 16–20 days of gestation. Within the extraembryonic (placental) compartment, nRBCs begin to populate villous capillaries of the developing embryo at ∼6 weeks postconception and remain prevalent until the end of the first trimester. Thereafter, their presence decreases so that at term, under normal circumstances of delivery, nRBCs are inapparent upon cursory examination in the placental fetal vasculature. Readily identifiable nRBCs are present on placental examination at term in a number of clinical circumstances and in conjunction with a variety of placental processes. Highlights are considered below.