Placenta Previa is a serious condition in which the placenta attaches at the base of the uterus, blocking the cervical opening. In the picture to the left, you can see how there are different types of placement that are called "previa": a low-lying placenta, which doesn't interfere with the cervix, a partial placenta previa, which is more serious, and a complete previa, which fully covers the cervical opening and requires a surgical delivery. If you are diagnosed with previa, you may be asked to go on bedrest in order to avoid dangerous bleeding. You almost certainly will be required to go on "pelvic bedrest," and to avoid all penetration until after delivery.
Diagnosis of placenta previa before week 20 can often be premature. In fact, one study found that 97% cases of early diagnosis do not end up being a case of complete previa by the date of delivery. Another study has found, however, that an overlap of 25 mm or more at 20-23 weeks seems to be incompatible with later vaginal delivery. (see studies below)
An overlap of 25 mm or more at 20-23 weeks seems to be incompatible with later vaginal delivery.
The relevance of placental location at 20-23 gestational weeks for prediction of placenta previa at delivery: evaluation of 8650 cases.
Becker RH, Vonk R, Mende BC, Ragosch V, Entezami M
Ultrasound Obstet Gynecol 2001 Jun;17(6):496-501