Placental Symmetry and Persistent Placenta Previa

This study indicates that placentas that are centered over the cervix in the second trimester are most likely to persist. Also worth noting, only 22 of the 43 women whose previa persisted actually bled. (Just less than half.)

It says ‘most likely’, but I still don’t understand exactly HOW MANY of the original group with centered previas persisted. (What does sensitivity of 49% mean?)

Second trimester sonographically diagnosed placenta previa: prediction of persistent previa at birth.

Zelop CC, Bromley B, Frigoletto FD Jr, Benacerraf BR.


Department of Obstetrics and Gynecology, Brigham & Women’s Hospital, Harvard Medical School, Boston, MA.



We sought to determine the natural history of second trimester sonographically diagnosed placenta previa, and to ascertain whether the position of the placenta with respect to the cervical os at second trimester sonography can accurately predict persistence of the placenta previaat term.


The study population included all women consecutively diagnosed by ultrasound with placenta previa between 14 and 20 weeks’ gestation. Medical records of the patients requiring cesarean section were reviewed to determine the presence of placenta previa. We reviewed the second trimester sonogram of patients who underwent abdominal delivery for placental and nonplacental indications to determine whether the central (symmetrical) versus the partial (asymmetrial) position of the placenta with respect to the internal os predicted the presence of placenta previa at delivery.


A total of 925 second trimester- patients were sonographically identified as having placenta previa. Two hundred and sixty seven patients underwent cesarean delivery, 43 of which had placenta previa (43/925 or 4.6%). Twenty-two of the 43 were asymptomatic without antepartum bleeding. Analysis of the second trimester position of the placenta revealed that symmetry of the placenta with respect to the internal os at second trimester scan had a sensitivity of 49% for prediction of placenta previa at birth.


The degree of placental symmetry with respect to the internal os during the second trimester successfully predicted the previas most likely to persist at delivery with a sensitivity of 49% (95% CI 34-64) and specificity of 93%.


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