Predicting pulmonary hypoplasia using the sonographic fetal lung volume to body weight ratio--how precise and accurate is it?

OBJECTIVES: To determine the precision and accuracy of ultrasound in estimating the fetal lung to body weight ratio (FLB ratio) using two-dimensional (2D) and three-dimensional (3D) ultrasound by comparison with postmortem measurements, and to evaluate its potential to diagnose pulmonary hypoplasia.

METHODS: Lung volumes were estimated by 3D ultrasound (rotational technique) and fetal weights were measured by 2D ultrasound (Hadlock equation) in 35 fetuses immediately before termination of pregnancy at 15-38 weeks. Sonographic estimates of FLB ratio were compared with postmortem values. Based on the pathological definition of pulmonary hypoplasia, the accuracy of sonographic estimation of the FLB ratio was analyzed.

RESULTS: The mean gestational age at termination of pregnancy was 26.7 (range, 15-38) weeks. The mean FLB ratios were 0.018 (SD, 0.006) on ultrasound and 0.019 (SD, 0.007) at autopsy (P = 0.730). Bias and precision of sonographic FLB ratio were - 0.001 and 0.003 (absolute limits, - 0.007 to + 0.006), respectively. Pulmonary hypoplasia was diagnosed in 12 (34.3%) cases at autopsy. The sonographic FLB ratio was significantly lower in fetuses with pulmonary hypoplasia at autopsy (median, 0.011; range, 0.004-0.014) than it was in those without pulmonary hypoplasia (median, 0.022; range, 0.013-0.045, P < 0.001). The sensitivity and specificity of the sonographic FLB ratio for diagnosing pulmonary hypoplasia were 91.7% (11/12) and 91.3% (21/23), respectively, the positive and negative predictive values were 84.6% (11/13) and 95.5% (21/22), and the accuracy was 91.4% (32/35).

CONCLUSION: FLB ratio can be estimated precisely on ultrasound examination, albeit with wide limits of agreement. The sonographically estimated FLB ratio may be useful in the prediction and diagnosis of pulmonary hypoplasia.