An evaluation of routine midtrimester ultrasonic scanning.

In a 2-year period 1,987 patients had a combined ultrasonic A and B scan between 16 and 30 weeks' gestation. The ultrasonic assessment of gestational age was shown to be more accurate than a definite menstrual history in 19,5% of patients using parameters of birth weight for gestation and the time of onset of spontaneous labour. All 31 sets of twins in the series were diagnosed at the first scan. There were 2 perinatal deaths among the twins, a perinatal mortality of 3,2%. Four unsuspected anencephalic fetuses were diagnosed and these pregnancies were terminated before 28 weeks. Two unsuspected molar pregnancies were diagnosed and evacuated. There were 58 false-positive diagnoses of placenta praevia, and 2 cases were missed. The situation of the placenta as revealed by ultrasound bore no relationship to abruptio placentae, but there was a significant relationship between a low-lying placenta and a history of threatened aboriton.