The placenta forms in the early stages of pregnancy and connects the developing fetus to the uterine wall of the mother. The normal term placenta measures 15-20 cm in diameter, 1.5-3 cm in thickness and weighs approximately 450-600 grams. The main components are the villous parenchyma and maternal decidual tissue, the umbilical cord (54-61 cm in length, containing two umbilical arteries and one umbilical vein) and placental membranes. The placental membranes include the amnion (the innermost lining of the amniotic cavity composed of a single layer of flat epithelial cells) and the chorion (connective tissue that carries the fetal vasculature).
The placenta is a highly vascularized organ, heavily infused with blood vessels. To maximize the contact area between maternal blood and placental villi, the so called chorionic villi develops from the outermost fetal membrane. The structure of the villus changes dramatically over the period of a normal gestation. Immature first trimester villi are large and covered by two layers of trophoblast, an inner layer of cytotrophoblasts and an outer layer of syncytiotrophoblasts. The villous stroma is very loose and blood vessels are small and centrally placed. Hofbauer cells (placental macrophages) are numerous. Second trimester villi are smaller, the syncytiotrophoblast layer is thinner and the nuclei are less evenly dispersed. The cytotrophoblast layer is discontinuous and difficult to identify, the villous stroma is more compact and collagenized. At this point the villous capillaries are larger, more numerous and located peripherally. Third trimester villi are smaller still with syncytiotrophoblastic cells arranged in tight knob-like structures. The cytotrophoblast layer is virtually gone and unidentifiable. The stroma is reduced to thin strands compressed between dilated fetal capillaries. The capillaries fuse with the thinned syncytiotrophoblast layer forming vasculosyncytial membranes where exchange between fetal and maternal blood circulation takes place.
The decidua is present both in the placental disc (from which it may be denuded during delivery) and on the chorionic side of the membranes. The decidua is composed of decidual cells that are derived from uterine stromal cells and is essential for the progress of implantation and establishment of fetal-maternal communication.