If the placenta abuts or overlies the os at the 18 – 20⁺⁶ week anomaly scan, management to identify those women who end up with placenta praevia (PP) is well established¹. We could find no evidence of the reverse side of the issue: whether a normal placental site at the anomaly scan could be relied upon to exclude a diagnosis of PP later in pregnancy. Amongst clinicians in our region, there was widely differing opinion.

Between April 2003 and February 2012 there were approximately 32500 deliveries in NHS Fife². 121 (0.37%) women were identified who had been delivered by caesarean section where the indication was PP. The placental site from the anomaly scan was identified using the local ultrasound reporting system. Caldicott approval was granted.

There were 59 (49%) emergency and 62 (51%) elective caesarean sections. 111 (92%) had an anomaly scan between 18 and 20⁺⁶ weeks.

In 46/111 women (41%, 95% CI 32–51%) the anomaly scan did not suggest the possibility of a problem. Hence, 3 – 5 out of every 10 women with a normally sited placenta at their anomaly scan may ultimately develop PP. We believe that women who present later in pregnancy should have a further scan to exclude PP even if the placenta was not significantly low at the anomaly scan.

References

1. Royal College of Obstetricians and Gynaecologists. Placenta praevia, placenta praevia accreta and vasa praevia: diagnosis and management. Green top guideline no. 27, January 2011.

2. Information Services Division Scottish Morbidity Record; http://www.isdscotland.org