Definition. Implantation of the placenta in the lower uterine segment over or near the internal cervical os; it is in front of the presenting fetal part constituting an obstruction to vaginal delivery.

Clinical Associations. Maternal cigarette smoking, multigravida, increasing maternal age, prior cesarean section or other uterine surgery, uterine leiomyoma; 10% are associated with abruptio placentae.

Gross Features. Cannot be diagnosed grossly. May see:

  • circumvallate placenta;
  • larger surface area placenta;
  • abnormal cord insertion;
  • disruption and hemorrhage at the placental edge, on the fetal and/or maternal surfaces;
  • old blood clots which may be laminated and brown, friable loose blood to partly decomposed material sometimes green from hemosiderin over the portion of villous tissue overlying the cervical os;
  • marginal atrophy or infarcts.

Microscopic Features. No unusual changes.

Etiology. Uneven blood flow to the endometrium might explain why leiomyomas, old surgical scars, cigarette smoking and maternal age are risks for placenta previa. Cigarette smoking and advanced maternal age are associated with marked vascular sclerosis of small myometrial arteries and arterioles. Because such lesions are unevenly distributed, blood flow could be restricted to some but not all of the endometrium. This may result in a placenta with a larger surface area, having grown wider in order to obtain an adequate blood supply. The poorly perfused endometrium associated with leiomyomas and old surgical scars may be inadequate for implantation, forcing the blastocyst to implant lower than normal in the uterus.

Significance. Placenta previa is the most frequent cause of third trimester bleeding and is life threatening for mother and fetus. The bleeding presumably arises from villous disruption at the placental edge when the cervix begins to dilate in late gestation. Placenta previa discovered in early pregnancy may resolve due to trophotropism; however, when the placenta initially completely covers the cervical os, the condition usually persists.

Risk. Placenta previa is associated with an increased risk for preterm birth, neonatal death, and neurological abnormalities including cerebral palsy and lesser motor abnormalities accompanied by mental retardation.