Measurement of the fetal biparietal diameter by ultrasound is not an accurate method of detecting fetal growth retardation.

Seventy-one patients had serial ultrasound examinations performed during the third trimester of pregnancy. Diameter, circumference and area measurements were made of the fetal head, thorax and abdomen. In three of five infants with birthweights below the 10th percentile no biparietal diameter measurements were below the 10th percentile whereas all five infants had at least the last two abdominal circumference measurements below the 10th percentile. The correlation coefficient between the various fetal parameters, measured within two weeks of delivery and birthweight averaged 0.83 for the thorax measurements and 0.85 for the abdomen measurements but was only 0.60 for the biparietal diameter. It is recommended that fetal abdominal measurements should be used for diagnosing growth retardation and not biparietal diameter measurements.