Purpose of review
Despite the widespread and routine use of ultrasound to make the diagnosis of placenta previa, evidence-based classification and management strategies have failed to evolve over the years. The purpose of this review is to present the current evidence supporting the screening, diagnosis and management of placenta previa.
The prevalence of placenta previa is significantly overestimated due to the practice of routine mid-pregnancy scan, and many women currently undergo a repeat scan in late pregnancy for placental localization. Recent reports support limiting third-trimester scans to only those cases where the placental edge either reaches or overlaps the internal cervical os at 20-23 weeks of pregnancy. In some cases of mid-trimester placenta previa, the placental edge is more likely to ‘migrate’ than others, and it appears that ultrasound may be useful to predict this process. At term, women with placental edge within 2 cm of the internal cervical os require a Caesarean section for delivery, whereas an attempt at vaginal birth is appropriate if this distance is more that 2 cm. Ultrasound also has a role in the diagnosis and management of both vasa previa and placenta accreta.
This review addresses screening for placenta previa. A simple and pragmatic ultrasound classification of placenta previa and low-lying placenta is proposed. Caesarean section is recommended for delivery in cases of placenta previa. Women with a low-lying placenta have at least 60% chance of a vaginal birth, but should be monitored for post-partum haemorrhage. Vasa previa is a rare complication but antenatal diagnosis is possible. It should particularly be suspected in in-vitro fertilization conceptions, and where the placental edge covers the os in mid-pregnancy but recedes later on. Prenatal diagnosis of placenta accreta should be based on the placental lacunae signs rather than the absence of retro-placental clear space.