A prospective study was performed on 98 patients to determine the risk of a second trimester placenta praevia remaining until term. The presence and degree of anterior or posterior praevia were scored for each patient. Follow-up data were obtained at delivery on 58 patients. A greater fraction (63.8%) of second trimester praevia was implanted posteriorly. The overall incidence of a second trimester praevia persisting until term was 8.8%.