Placental edge to internal os distance in the late third trimester and mode of delivery in placenta praevia

Objectives To correlate transvaginal ultrasound findings with mode of delivery in cases of placenta praevia. Design Cohort study. Setting A London Teaching Hospital. Methods Retrospective review of all cases of placenta praevia diagnosed by transvaginal ultrasound between February 1997 and March 2002. Main outcome measures Likelihood of vaginal delivery and major obstetric haemorrhage. Results A total of 121 pregnancies were studied with a mean scan-to-delivery interval of 10.5 days. In the 64 women who laboured, the likelihood of vaginal delivery rose significantly as the placental edge to internal os distance increased. Caesarean section rate was 90% when the placental edge-internal os distance was 0.1 to 2.0 cm, falling to 37% when this measurement was over 2.0 cm (P < 0.00045). Conclusion Trial of vaginal delivery is appropriate in cases with a placental to internal os distance >2 cm. The term 'praevia' should be restricted to cases where the placental edge is <2 cm from the internal os, as the likelihood of operative delivery and significant postpartum haemorrhage is high. Cases where the placenta is more than 2 cm from the internal os have a greater than 60% chance of vaginal delivery and should be defined as 'low lying' in order to reduce the clinician's bias towards operative delivery.