The placenta, also called the afterbirth, is the organ that develops during pregnancy between the mother and the fetus. The placenta has many blood vessels that allow oxygen and food to flow from the mother to the fetus, and remove  waste products from the fetus.

If the placenta covers the cervix, it is called placenta previa. Placenta previa does not appear to affect the development of the fetus, but may cause the fetus to grow slightly smaller than average and does prevent the fetus from being delivered vaginally

As pregnancy progresses, the placenta tends to grow towards the upper uterus and away from the cervix.  Therefore, a follow-up ultrasound is recommended to be done at 32 weeks to re-examine the location of the placenta. If the placenta is still covering the cervix at that time, then another exam is usually done at 36 weeks. If the placenta previa persists, then delivery by cesarean section at 36 0/7 to 37 6/7 weeks is recommended.