One hundred and fifty-six mid-trimester sonograms were performed at our prenatal diagnostic unit. Twenty women were found to have a low-lying placenta or placenta previa and were followed by serial ultrasound examinations to observe changes in placental position. Eighty percent of women, i.e., 16/20, with a low-lying placenta had converted to normal implantation by the time of delivery. Most of the conversions had taken place at approx. 34 weeks of gestation. The patients with mid-trimester low-lying placenta had an increased risk of third-trimester bleeding, abruptio placentae and cesarean sections. The infants were also at risk of premature delivery. Patients with mid-trimester low-lying or placenta previa should be followed by ultrasound to monitor delivery.