From low-lying implantation to placenta praevia: a longitudinal ultrasonic assessment.

A study of 895 consecutive pregnant women studied by transabdominal ultrasound scan showed an incidence of 14.6% of low-lying placenta at 12-14 weeks' gestation. Longitudinal follow-up of those with low-lying placenta showed that 85% of them had normally situated placenta at term. However, for those with partial or total placenta praevia at 30 weeks' gestation, 60% and 75%, respectively, persisted as major placenta praevia at term. A total of 45% of those with low-lying placenta at recruitment had threatened abortion while 15.7% had antepartum haemorrhage. The caesarean section rate was as high as 26% among the study population compared with 8.4% among the general population (P < 0.005). It is recommended that all antenatal women should have at least one ultrasound scan in the second or third trimester for placental localisation. Those found to have major placenta praevia in the third trimester should be closely followed-up.