What is placenta previa?

Placenta previa means that your placenta is lying unusually low in your uterus, next to or covering your cervix. The placenta is the pancake-shaped organ--normally located near the top of the uterus--that supplies your baby with nutrients through the umbilical cord. With placenta previa, the placenta is attached to the uterine wall close to the cervix. Placenta previa affects about 1 in 200 pregnant women in the third trimester of pregnancy.

When the placenta is in position in the uterus lower down than the baby, it is in position to come out first during delivery. The placenta may separate from the uterine wall as the cervix begins to dilate (open) during labor. If you have placenta previa when it's time to deliver your baby, you will need a cesarean section. 

The location of your placenta can be checked during your midpregnancy ultrasound examination. Since the placenta is implanted in the uterus, it does not actually move, but it can end up farther from your cervix as your uterus expands.  If the placenta covers the cervix completely, it's called a complete or total previa. If the placenta is right on the border of the cervix, it's called a marginal previa. If the edge of the placenta is within 2 centimeters of the cervix but not bordering it, it's called a low-lying placenta.

The most common symptom of placenta previa is painless bleeding during the third trimester. Bleeding from placenta previa happens when the cervix begins to thin out or dilate (even a little) and disrupts the blood vessels in that area.

The primary risk of placenta previa is massive hemorrhage and preterm delivery.  Placenta previa is a leading cause of death from pregnancy, but it can be discovered by sonogram and medically managed.