This chapter discusses placental, fetal and neonatal circulation, where uterus shows cyclic changes in blood flow during the menstrual cycle. Uterus receives blood through uterine arteries. The exposed ends of damaged arteries bleed, leading to menstruation. The cyclic changes indicate that uterine blood flow is dependent on the female reproductive hormones, estrogen and progesterone. The placenta provides the circulatory link between the mother and the fetus. Free mixing of fetal and maternal blood is prevented by fetal vascular endothelium, connective tissue of the villi and the trophoblast. Uterine blood flow increases enormously during pregnancy. Just before parturition uterine blood flow decreases. Ductus arteriosus remains patent for many hours to many days after birth. The fetus receives its blood supply through the umbilical vein which carries blood from the placenta to the fetus via the umbilical cord. Reduction in pulmonary blood flow limits the capacity for oxygenation of blood.