The placenta is the organ that connects a mother to her unborn child or fetus. The placenta transfers oxygen and other nutrients from the mother's bloodstream to the fetus. It also carries waste products away from the fetus.

Normally, the fertilized ovum implants itself in the upper portion of the uterus. If the implantation occurs in a lower position in the uterus (toward the opening of the uterus, called the cervix), there is a possibility that the placenta will grow and eventually block the cervix. When this occurs, it is referred to as placenta previa.

The condition is highly variable since the placenta may only partially block the cervix, or it may completely cover it. During normal labor, the cervix opens up (dilates) to allow the baby to pass through the birth canal. In the setting of placenta previa, the process of cervical dilation can cause the placenta to tear and bleed.

Placenta previa can be a serious cause of vaginal bleeding in women who are 24 to 28 weeks pregnant. It may occur anytime after the 20th week of pregnancy.