FL/AC ratio: poor predictor of intrauterine growth retardation

Early antenatal detection of intrauterine growth retardation (IUGR) may decrease the associated perinatal morbidity and mortality. A parameter based on sonographically measured femur length (FL) and abdominal circumference (AC), expressed as FL/AC X 100 and termed the FL/AC ratio, has recently been proposed by Hadlock et al as an age-independent predictor of IUGR. We studied 285 normal and 37 IUGR fetuses to verify that the FL/AC ratio is independent of gestational age (GA) and to assess its value as a predictor of IUGR. Our results confirm that the FL/AC ratio is age-independent above 20 weeks and that its mean value differs in normal (22.4 +/- 1.7) and IUGR (23.7 +/- 1.4) fetuses (P less than .01, t-test). Because of considerable overlap between these two groups, however, there is no cutoff value for the FL/AC ratio that yields both a high sensitivity and a high specificity, or that leads to a high positive predictive value. With a cutoff of 23.5, for example, the sensitivity is 56% and the specificity 74%, and, even assuming an IUGR prevalence rate of 10%, the likelihood of IUGR in a fetus with an FL/AC ratio above the cutoff is only 19%. We conclude that the FL/AC ratio, though an age-independent measure whose mean value differs in normal and IUGR fetuses, is not clinically useful as a predictor of IUGR.