The mid-trimester low lying placenta: a prospective study.

Thirteen women who were found to have a low lying placenta on ultrasound scan during the mid-trimester were followed prospectively with repeated ultrasound examinations in older to observe changes in placental position and to determine the prognostic significance of low-lying placentae found during the mid-trimester. Two women aborted spontaneously and in the remainder the placenta ceased to be previa by 30 to 32 weeks gestation ie about the time of the formation of the lower uterine segment.