Transabdominal ultrasonography as a screening test for second-trimester placenta previa

To determine the test characteristics of transabdominal ultrasonography as a screening test for second-trimester placenta previa.

This secondary analysis of a prospective cohort study evaluated the distance from the placental edge to the internal os (placenta-cervix distance) through both transabdominal and transvaginal ultrasonography during the anatomic survey. Patients were recruited in the Maternal-Fetal Medicine Ultrasound Unit at the Hospital of the University of Pennsylvania, an urban tertiary care center. Transabdominal placenta-cervix distance cutoffs with high sensitivity for detection of previa and low-lying placenta were identified, and test characteristics were calculated. Follow-up ultrasound data, pregnancy, and delivery outcomes for those with second-trimester previa or low-lying placenta were obtained.

One thousand two hundred fourteen women were included in the analysis. A transabdominal placenta-cervix distance cutoff of 4.2 cm was 93.3% sensitive and 76.7% specific for detection of previa with a 99.8% negative predictive value at a screen-positive rate of 25.0%. A cutoff of 2.8 cm was 86.7% sensitive and 90.5% specific with a 99.6% negative predictive value at a screen-positive rate of 11.4%. Only 9.8% (four of 41) of previas and low-lying placentas persisted through delivery.

Transabdominal ultrasonography is an effective screening test for second-trimester placenta previa. At centers not performing universal transvaginal ultrasonography at the time of the anatomic survey, evidence-based transabdominal placenta-cervix distance cutoffs can optimize the identification of patients who require further surveillance for previa.