A decidual cast occurs when the uterus sheds its lining during pregnancy. The lining has a thick outer layer, allowing it to emerge intact from the body in the shape of a cast of the uterus.
Decidual casts are often the result of ectopic pregnancies. In these instances, an embryo implants outside the decidua, either on the exterior wall of the uterus or somewhere else in the abdominal cavity. When the fetus begins to develop, hormones trigger the body to begin producing decidua. However, when the fetus does not begin to develop inside the uterus, the body spontaneously sheds the lining.