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

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 midpregnancy, reducing anxiety and resource utilisation.