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?

Introduction: 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.