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

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.

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.

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.

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).

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.