placenta previa /prē″vē·ə/ , (Informal) a condition of pregnancy in which the placenta is implanted abnormally in the uterus so that it impinges on or covers the internal os of the uterine cervix. It is the most common cause of painless bleeding in the third trimester of pregnancy. Its cause is unknown. The incidence of the condition increases with increased parity from approximately 1 in 1500 primiparas to approximately 1 in 20 grand multiparas. Even slight dilation of the internal os can cause enough local separation of an abnormally implanted placenta to result in bleeding. If severe hemorrhage occurs, immediate cesarean section is usually required to stop the bleeding and to save the mother’s life; it is performed regardless of the stage of fetal maturity. Before hemorrhage, placenta previa may be diagnosed by ultrasonography and treated with complete bed rest under close observation. Even at rest sudden massive hemorrhage can occur without warning. Vaginal examination is usually contraindicated if placenta previa is present or suspected because palpation can cause local placental separation and precipitate hemorrhage. Cautious and very gentle intracervical palpation may be performed to determine the existence and exact extent of placenta previa. Before this examination an IV infusion is begun, the woman’s blood is typed and crossmatched, and preparations for immediate cesarean section are made. If the placenta is next to or near, rather than touching or covering, the cervical os, labor and vaginal delivery may be attempted. Central placenta previa refers to a placenta that has grown to cover the internal cervical os completely; low-lying placenta identifies a placenta that is just within the lower uterine segment; and partial or marginal placenta previa is a condition in which the placenta partially covers the internal cervical os. Also called previa, placental presentation. Compare abruptio placentae.
