Objectives: The purpose of this study was to determine how often a low-lying placenta, defined as a placenta ending within 2 cm of the internal cervical os but not covering it, diagnosed sonographically in the second trimester resolves before delivery.
Methods: After Institutional Review Board approval was obtained, 1416 pregnancies with a sonographically diagnosed low-lying placenta between 16 and 24 weeks' gestation were identified from our ultrasound database over a 5-year period. We reviewed medical records to determine the gestational age at which the low-lying placenta was first diagnosed, the gestational age at which the placenta was no longer sonographically low lying or covering the cervix, and, of those whose placentas that never cleared the internal cervical os sonographically, how many went on to cesarean delivery as a result of placental location.
Results: In total, 1220 of 1240 low-lying placentas (98.4%) that had sonographic follow up resolved to no previa before delivery; 89.9% of placentas cleared the cervix by 32 weeks, and 95.9% cleared by 36 weeks. Twenty patients (1.6%) had persistent sonographic placenta previa or a low-lying placenta at or near term, including 5 complete previas, 7 marginal previas, 5 low-lying placentas, and 3 vasa previas; all had cesarean deliveries.
Conclusions: A low-lying placenta sonographically diagnosed in the second trimester typically resolves by the mid third trimester. Only rarely (1.6% of the time) does it persist to term or near term. Follow-up sonography is warranted to diagnose persistent placenta previa or vasa previa, a complication of a low-lying placenta.
Keywords: low-lying placenta; obstetric ultrasound; placenta previa; sonography; vasa previa.