Placenta previa is a condition that occurs during pregnancy when the placenta lies low in the uterus and partially or completely covers the internal cervical os (the small opening of the cervix that dilates during the first stage of labor).

Shortly after conception, the placenta (a flat, circular organ) attaches to the inner wall of the uterus and the baby’s umbilical cord. It transfers oxygen and nutrients from the mother’s blood to the fetus. It also transfers waste from the baby’s blood to the mother’s blood, where the mother’s kidneys dispose of the waste. The placenta also protects the baby from infection and produces hormones that trigger labor and delivery.

Under normal circumstances, the placenta migrates away from the mother’s cervix as the uterus grows. In women with placenta previa, this does not happen. The abnormally positioned placenta can block the birth canal, obstructing the baby’s exit from the uterus. As the cervix dilates and thins in preparation for labor, blood vessels connecting the placenta to the uterus can tear and cause bleeding.

The bleeding can become severe and pose serious complications to the mother and baby, including:

• Infection. The risk of infection to the mother and baby increases with placenta previa.
  
  
• Blood clots (clumps that result from coagulation of the blood). Women with placenta previa are at a higher risk of getting blood clots.
  
  
• Premature birth (birth of an infant before 37 weeks of gestation). Placenta previa can result in premature labor and delivery.
  
  
• Massive bleeding (hemorrhaging). Placenta previa can cause massive vaginal bleeding due to the placenta separating from the uterine wall during labor. Women experiencing massive blood loss may require a blood transfusion. Hemorrhaging can also cause the mother or baby to go into shock (a life-threatening condition that occurs when the body is not getting adequate blood flow). In rare cases, it can result in the death of the mother or baby.

Placenta previa occurs in approximately 1 in 200 births, according to the National Institutes of Health (NIH).

There are several placental conditions that are related to placenta previa. They can also cause vaginal bleeding in the second and third trimesters. These conditions include:

• Placental abruption (placenta abruptio). A condition in which the placenta breaks away from the uterine wall. This condition can deprive the baby of oxygen and nutrients and cause heavy bleeding. It can be a complication of placenta previa, but usually occurs in women without it.
  

• Placenta accreta. A condition in which the placenta implants too firmly into the uterine wall. Placenta accreta is most common in women with placenta previa. It can cause vaginal bleeding and result in premature birth. This condition most commonly occurs in women who have had previous uterine surgery. Some women with placenta accreta need a hysterectomy, as this may be the only way to stop the massive bleeding that can occur with this condition.
  
  
• Vasa previa. A condition in which the umbilical cord develops in an abnormal position. This allows the baby’s blood vessels to cross the cervix, resulting in rupture. This can cause life-threatening bleeding in the baby.