Precise mid-trimester placenta localisation: does it predict adverse outcomes?

Abstract

Background: A low-lying placenta detected at the mid-pregnancy ultrasound is commonly reported to warn against potential morbidity associated with placenta praevia. There is no information on what distance away from the internal cervical os is safe.

Aims: We examined whether a low-lying placenta not overlapping the cervical os in the second trimester increases the risk of obstetric complications and whether there is a cut-off point at which that increase occurs.

Methods: Adverse perinatal outcomes were examined prospectively in a cohort of women with a placenta 0-30 mm from the internal cervical os ('low-lying') at the routine mid-trimester ultrasound and compared to those with a placenta further away. Two composite outcomes of 'major' and 'minor' adverse events were predefined as primary outcome measures, requiring a sample size of 480 women with a low-lying placenta. Chi-square and Fisher's exact tests were used for statistical analysis.

Results: In 1662 pregnancies ('low-lying': n = 484; 'normal': n = 1178), there was no increase in composite adverse outcomes with a low-lying placenta and no cut-off distance within 30 mm from the cervical os at which risks increased. Postpartum haemorrhage ≥ 1000 mL was more frequent with a low-lying placenta (7.6% vs 4.7%, P < 0.05).

Conclusions: Women with a low-lying placenta, not overlapping the cervical os, in mid-pregnancy are at no higher risk of adverse outcomes than those with a normally located placenta, except postpartum haemorrhage. This suggests that the high-risk label can be removed from pregnancies with a low-lying placenta not overlapping the cervical os in mid-pregnancy, reducing anxiety and resource utilisation.