Comparison of the fetogram and L/S ratio for fetal maturity

There were 254 cases studied in which two tests of fetal maturity were performed within 72 hours of each other. The two tests were an x-ray fetogram and an amniotic fluid L/S ratio determination. All fetograms were read by one author for the presence of distal femoral epiphysis (DFE). It was assumed that DFE permits estimation of fetal bone maturation and L/S ratio permits estimation of fetal lung maturity. The DFE results gave a 40% false-positive rate in predicting a mature L/S ratio (greater than 2.0) and a 38.7% false-negative rate. Of the 21 patients who delivered within 3 days of the performance of the two tests, all neonates did well when there were no visible DFE's but a mature L/S ratio. There is a significant lack of specificity and sensitivity of the fetogram DFE's in predicting a mature L/S ratio. Since infant lung maturation is a critical factor in predicting neonatal survival, these results suggest that the x-ray DFE is unacceptable as a single maturity test for use in a perinatal center managing complicated obstetric cases.