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. Doctors classify placenta previa according to the placenta's location on the uterine wall and how completely the placenta covers a woman's cervix:
- complete -the placenta covers the entire opening of the cervix. If the placenta is centered exactly over the cervical opening, it may be called a central previa;
- partial -the placenta covers a significant part of the cervix, but not the entire area; and
- marginal -the placenta reaches the edge of the cervical opening, but doesn't actually cover any part of it.
An additional form, similar to marginal placenta previa, is the low-lying placenta, in which the edge of the placenta extends to within two centimeters of the cervix.
About 30% of pregnant women have placenta previa early in their pregnancies; however, most of the time this spontaneously resolves as the pregnancy progresses. By the time a woman is within a couple of weeks of her due date, the chance of placenta previa drops to less than 1%. The reason placenta previa is less common in late pregnancy has to do with the way the uterus stretches over the nine months. The top half of the uterus tends to stretch more easily than the bottom half so that, as the baby grows, the uterus pulls the placenta up and away from the cervix. This is called placental migration.
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:
- history of cesarean section;
- previous pregnancy;
- previous dilation and curettage (scraping) for miscarriage or abortion; and
- maternal age 35 or older.
Previa may also develop when more surface is needed to provide oxygen. These situations include:
- twin or multiple births;
- maternal smoking; and
- living at higher altitudes.
Placenta previa is usually characterized by vaginal bleeding in the late second or third trimester. However, placenta previa often causes no symptoms throughout the pregnancy and is often 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, ultrasound may be conducted carefully through the vagina.
For the baby, preterm delivery (and the complications resulting from it) is the most common potential complication of placenta previa. Preterm delivery occurs in nearly two-thirds of placenta previa cases. Babies are usually delivered early because of dangerous amounts of bleeding. Complications stemming from the position of the baby during delivery occur in up to 30% of cases. Placental separation and bleeding due to placenta previa may cause anemia in the newborn.
For the mother, bleeding and complications during cesarean delivery are the most common potential complications of placenta previa. Blood transfusions are necessary in one-third to one-half of the cases. In addition, 9 to 10% 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, often necessitating hysterectomy. Placenta accreta is particularly common in women who have placenta previa and a history of one or more previous cesarean sections. More than 50% of patients with placenta accreta require blood transfusions.