With placenta previa, the placenta is implanted in the lower uterine segment, where it encroaches on the internal cervical os. This disorder, one of the most common causes of bleeding during the second half of pregnancy, occurs in approximately 1 in 200 pregnancies, more commonly in multigravidas than in primigravidas. Generally, termination of pregnancy is necessary when placenta previa is diagnosed in the presence of heavy maternal bleeding. Maternal prognosis is good if hemorrhage can be controlled; fetal prognosis depends on gestational age and amount of blood lost.
With placenta previa, the placenta may cover all (total, complete, or central), part (partial or incomplete), or a fraction (margin or low-lying) of the internal cervical os. (See Three types of placenta previa.) The degree of placenta previa depends largely on the extent of cervical dilation at the time of examination because the dilating cervix gradually uncovers the placenta. Although the specific cause of placenta previa is unknown, factors that may affect the site of the placenta’s attachment to the uterine wall include:
defective vascularization of the decidua
multiple pregnancy (the placenta requires a larger surface for attachment)
previous uterine surgery
advanced maternal age.
With placenta previa, the lower segment of the uterus fails to provide as much nourishment as the fundus. The placenta tends to spread out, seeking the blood supply it needs, and becomes larger and thinner than normal. For unknown reasons, eccentric insertion of the umbilical cord often develops. Hemorrhage occurs as the internal cervical os effaces and dilates, tearing the uterine vessels.
Signs and symptoms
Placenta previa usually produces painless third-trimester bleeding (typically the first complaint). Because of the placenta’s location, various malpresentations occur that interfere with proper descent of the fetal head. (However, the fetus remains active, with good heart tones.) Complications of placenta previa include shock or maternal and fetal death.
Special diagnostic measures that confirm placenta previa include:
transvaginal ultrasound scanning for placental position
pelvic examination, performed only immediately before delivery. In most cases, only the cervix is visualized.