Placenta previa is a condition in which the placenta attaches to the wall of the lower portion of the uterus and covers all or part of the cervix. This condition is divided into three categories:

  • Total Placenta Previa, in which the placenta completely covers the cervix;
  • Partial Placenta Previa, in which the placenta partially covers the cervix; and
  • Marginal Placenta Previa, in which the placenta extends to the margin of the cervix.

An additional form, similar to marginal placenta previa, is the low-lying placenta, in which the edge of the placenta extends to within two to three centimeters of the cervix.

Placenta previa is usually characterized by vaginal bleeding in the late-second or third trimester, possibly with pain in the uterus and/or contractions. But placenta previa often causes no symptoms throughout the pregnancy and is diagnosed only during a routine ultrasound. When an abdominal ultrasound does not allow the doctor to see the relationship between the placenta and the cervix, the ultrasound may be conducted carefully through the vagina.

Although the exact cause of placenta previa is unknown, the following risk factors suggest that some cases may be caused by previous scarring of the wall of the uterus:

  • previous cesarean section;
  • previous pregnancy;
  • previous scraping of the uterine lining during dilation and curettage procedure;
  • mother over age 35;
  • twin or multiple births;
  • maternal smoking;
  • residence at higher altitudes; or
  • erythroblastosis fetalis-a type of anemia caused by the conflict of Rh factors in the blood of the mother and fetus.

Approximately one in every 250 pregnancies results in placenta previa at term. The condition is much more common in early pregnancy. However, because the uterus grows during pregnancy, more than 90% of early placenta previa cases resolve without treatment by the time of delivery.

Potential Complications from Placenta Previa

For the baby, preterm delivery-and the complications resulting from it-is the most common source of injury. Preterm delivery occurs in nearly two-thirds of placenta previa cases. Complications stemming from the position of the baby during delivery are observed in up to 30% of cases. Placental separation and bleeding due to placenta previa may also cause anemia in the newborn.

For the mother, bleeding and complications during cesarean delivery are the most common causes of injury. Blood transfusions are necessary in one-third to one-half of cases. In addition, 1 to 5% of placenta previa cases are associated with placenta accreta, an abnormally firm attachment of the placenta to the wall of the uterus. Placenta accreta prevents the placenta from separating from the uterine wall at the time of delivery. This can cause severe bleeding, which often necessitates hysterectomy (removal of the uterus). Placenta accreta is particularly common in women with placenta previa and one or more previous cesarean sections (C-sections), and may complicate one-third to one-half of all such cases. More than 50% of patients with placenta accreta require blood transfusions.