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The placenta grows wherever the embryo implants itself in the uterus. If the embryo implants itself in the lower portion of the uterus, the placenta might grow over the cervix — causing placenta previa.

Most cases of placenta previa are diagnosed at the time of a second trimester ultrasound examination. If the placenta is just barely reaching the cervix, the situation will resolve itself because expansion of the uterus pulls the area of placental attachment higher up in the uterus, away from the cervical opening. If the placenta is found to be all the way across the cervix, however, it is unlikely to resolve with time.

Persistent types of placenta previa have been associated with:

  • Scars in the lining of the uterus
  • A large placenta, such as with a multiple pregnancy
  • An abnormally shaped uterus
Jun. 02, 2011