Placenta previa refers to implantation of the placenta in the lower segment of the uterus, so that it partially or totally covers the internal os of the cervix. Placenta previa and abruptio placentae account for more than 85% of the cases of hemorrhage during the last trimester of pregnancy. This is associated with potentially catastrophic maternal bleeding and obstruction of the uterine outlet.
Placenta previa is classiﬁed into four types, according to the degree with which the placenta superimposes or encroaches on the internal os: total or central placenta previa, in which the internal os is covered entirely; partial placenta previa, in which the placenta partially caps the internal os (10% to 90%); marginal placenta previa, in which only a small edge of the placenta is at the internal os; and low-lying, in which the placenta is located in the lower uterine segment but does not touch the cervical os. These degrees may vary with cervical dilation or gestational age.
Ultrasonographic studies indicate that the location of the placenta is subject to some degree of “migration” during the course of gestation. In addition, as the lower uterine segment elongates late in pregnancy, these relationships may change. Therefore, the above classiﬁcation of placenta previa is only relative, and it should be remembered that if a diagnosis is made of a particular type, it refers to the time of examination. In the partial and total varieties of placenta previa, a slight degree of separation of the placenta is inevitable when the lower segment of the uterus distends, and hence a certain degree of bleeding is bound to occur.
The incidence of placenta previa varies from 1 in 100 to 1 in 200 deliveries. The condition is much more frequent in multiparas than in primiparas, in older patients (older than 35: 1%; older than 40: 2%), with a prior cesarean delivery (two to ﬁvefold increase), in smokers (twofold increase), following in vitro fertilization, and in multiple gestations.
Little is known regarding the etiology of the condition. It has been suggested that defective vascularization of the decidua, as the result of inﬂammatory or atrophic processes, may be a contributing factor for placenta previa. Under these circumstances, the placenta is forced to spread over a wide area in order to obtain sufﬁcient blood supply. It is also possible that a multiplicity of factors contributes to lower implantation of the ovum with extension of the placenta toward the internal os.
The symptoms of placenta previa include painless hemorrhage (70% of cases), which usually appears after the seventh month of gestation. The hemorrhage may come at any time, without warning and even when the patient is asleep. It usually begins as a slight intermit- tent bleeding, but it may become profuse without any notice. The mechanisms of bleeding in placenta previa are poorly understood. Separation of small areas and tears in the vessels may occur as the consequence of stretching of the uterine walls, especially the distended lower segment. The blood is maternal in origin.
The diagnosis is usually not difﬁcult when the classic symptoms are present. Ultrasonography has replaced other imaging techniques and the classic “double set-up” (vaginal examination in the operating room so that an emergency operative delivery could be accomplished should hemorrhage be precipitated). It is important to remember that any vaginal manipulation may precipitate extensive hemorrhage.
Because of the overstretched lower segment and abnormalities of placental attachment, profuse bleeding may occur even after the delivery of the fetus. The lower segment may be unable to contract sufﬁciently to check the bleeding. Placenta accreta occurs in 15% to 25% of cases of placenta previa, particularly in the presence of a previous cesarean section scar.