Femur length/abdominal circumference ratio. Poor predictor of macrosomic fetuses in diabetic mothers

Antenatal diagnosis of fetal macrosomia can affect the management of diabetic mothers. Because the sonographically determined femur length/abdominal circumference (FL/AC) ratio has been shown to differ in macrosomic and non-macrosomic fetuses (in a population containing few diabetics), its value in establishing the diagnosis of macrosomia was examined. The results indicate that the FL/AC ratio differs in non-macrosomic (20.4 ± 1.6) and macrosomic (19.5 ± 1.4) fetuses of diabetic mothers, but there is considerable overlap. For no cutoff value is there a high sensitivity and high specificity. The positive predictive value is 36-43 per cent (depending on the cut-off), only slightly greater than the prevalence (26 per cent in the study population). The authors conclude that the FL/AC ratio is not useful in predicting macrosomia among diabetic mothers.