Effectiveness of ultrasound determination of fetal abdominal circumference and fetal ponderal index in the diagnosis of asymmetrical growth retardation.

A total of 310 unselected women attending an antenatal clinic was screened for growth retardation by ultrasound between 34 and 36 weeks gestation, by measuring the fetal abdominal circumference (AC) and femoral length (FL), from which the 'fetal ponderal index' (AC/FL) was calculated. Asymmetrical growth retardation in the newborn was assessed by Rohrer's ponderal index and the mid-arm/occipito-frontal circumference (MAC/OFC) ratio within 72 h of birth, a neonatal ponderal index or MAC/OFC ratio below the 10th centile being considered abnormal. The sensitivities of an AC below the 25th centile in identifying a birthweight, neonatal ponderal index or MAC/OFC ratio below the 10th centile were 86, 62 and 67% respectively, the specificities being 80, 78 and 76%. The sensitivities of a fetal ponderal index below the 25th centile in identifying a neonatal ponderal index or MAC/OFC ratio below the 10th centile were 52 and 47% respectively, the specificities being 77 and 77%. A possible reason for the poor performance of the fetal ponderal index is discussed.