Measuring leading placental edge to internal cervical os: Transabdominal versus transvaginal approach

We aimed to compare the value of transabdominal ( TA ) and transvaginal ( TV ) approaches for assessing the risk of a low‐lying placenta. This involved a comparison of TA and TV measurements between the leading placental edge and the internal cervical os. We also assessed the intra‐/interobserver variation for these measurements and the efficacy of TA measures in screening for a low placenta. Methodology Transabdominal and TV measurements of the leading placental edge to the internal cervical os were performed on 369 consecutive pregnancies of 16–41 weeks' gestation. The difference ( TA ‐ TV ) from the mean was calculated and plotted against gestational age. Bland‐Altman plots and paired t‐tests were used to look at the differences in TA / TV measurement. Screening performance of a transabdominal approach was compared to a transvaginal ‘gold standard'. Nonparametric methods were used to calculate the area under the receiver operator characteristics ( ROC ) curve. Intra‐/interobserver variations were also calculated. Results Of the pregnancies, 278 had a leading placental edge that was visible with the TV approach. Differences ( TA ‐ TV ) ranged from −50 mm to +57 mm. Bland‐Altman plot shows that TA measurements overestimated the distance compared with the TV measurements; the average difference in measurement was 12.0 mm (95% confidence interval 9.9–14.1). The sensitivity, specificity and negative predictive values of a TA approach were 18.2%, 97.5% and 87.2%, respectively. The receiver operator characteristics area between gestational weeks 16–23 was 0.81 (95% CI : 0.76–0.86). Conclusion The TA approach has a low sensitivity for detecting a low‐lying placenta as choosing a TA cut‐off with sensitivity >90% will decrease the specificity to 50%.