(HealthDay News) -- Placenta previa occurs when, during pregnancy, the placenta drops lower in the uterus and covers the opening to the cervix. The placenta may also separate from the uterine wall as the cervix begins to dilate.

Placenta previa affects about one in 200 pregnant women in the third trimester, the American Pregnancy Association says.

The association lists these common risk factors for placenta previa:

• Having had one or more previous deliveries.
• Having had a previous Caesarean section.
• Having had a multiple birth (twins or triplets).
• Having had prior surgery on the uterus.