Is the third trimester repeat ultrasound scan for placental localisation needed if the placenta is low lying but clear of the os at the mid‐trimester morphology scan?

It is unclear whether women with a low‐lying placenta (not overlapping the internal cervical os) at the mid‐trimester scan need follow‐up. The aim of the study was to determine the rate of placenta praevia, vasa praevia and cord prolapse in the third trimester in this cohort of women. Methods A retrospective cohort study of women with a documented low‐lying placenta (<30 mm from the internal cervical os) at the mid‐trimester morphology ultrasound scan was done. A composite outcome of rate of placenta praevia, vasa praevia or cord prolapse at term was evaluated. Multivariate analysis was performed to investigate the variables influencing the composite outcome. In addition, a meta‐analysis of methodologically similar studies was performed to investigate the effect of the placenta to os distance at the mid‐trimester scan on the rate of placenta praevia at term. Results One hundred and eighty‐one women with a low‐lying placenta not overlapping the os at mid‐trimester scan were identified. The composite outcome was documented in 20 (11.0%) women, including placenta praevia in 15 (8.3%). Based on multivariate analysis, multiparity, distance from os < 10 mm and antenatal bleeding were independently associated with the composite outcome. Meta‐analysis demonstrated significant reduction in rate of placenta praevia for every 10‐mm increase in placenta–os distance at mid‐trimester. Conclusion Our study supports the recommendation to follow up all women with a placenta lying <20 mm from the cervical os at mid‐trimester.