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Transvaginal ultrasound prior to 20 weeks identifies placenta previa in 1/20 pregnancies, but only about 10% persist in the third trimester. Prior studies have associated decreased cervical length (CL) in the setting of placenta previa with adverse obstetric outcomes including maternal hemorrhage, preterm birth, emergency cesarean and abnormally adherent placenta. However, the association between CL and persistence of placenta previa has not been evaluated. We sought to test the hypothesis that cervical shortening with associated development of the lower uterine segment in the setting of placenta previa is associated with impaired placental migration away from the internal cervical os, and persistent placenta previa.
A retrospective cohort study of singleton pregnancies presenting for routine fetal anatomic survey (17w0d- 23w6d). Women with multiple gestations, major uterine anomalies and those without third trimester follow up ultrasound were excluded. The primary outcome was persistence of placenta previa on ultrasound in the third trimester (28w0d-36w6d). CL at the time of the anatomic survey in women with persistence and resolution of placenta previa in the third trimester were compared. The predictive value of second trimester CL for persistent placenta previa in the third trimester was assessed using the receiver-operating characteristics (ROC) curve.
294 women diagnosed with placenta previa at anatomic survey in the second trimester met inclusion criteria. Of these, 16 (5.4%) had placenta previa on follow-up ultrasound in the third trimester. CL was not significantly different in women with persistent placenta previa compared to those with resolution (45.1±8.8mm versus 43.4±8.2mm, p=0.42). The area under the ROC was 0.58, and no CL cutoff was significantly associated with persistence of placenta previa (Table).
These data suggest that second trimester CL is not predictive of persistence of placenta previa in the third trimester.