Purpose: To retrospectively determine an algorithm based on fetal body volume (FBV) by using magnetic resonance (MR) imaging to calculate relative lung volume in fetuses with normally developed lungs and prospectively assess the use of this algorithm in predicting pulmonary hypoplasia in the late second and early third trimesters for fetuses at risk for pulmonary hypoplasia. Materials and Methods: Oral informed consent was obtained for the prospective component of this ethics committee-approved study. MR imaging lung volumetry was performed in 36 fetuses with normally developed lungs between 18 and 39 weeks gestational age by using T2-weighted single-shot fast spin-echo imaging in fetal transverse and sagittal planes. Findings were then correlated with biometric variables and gestational age. The best-performing algorithm was applied to 37 fetuses (between 18 and 29 weeks gestational age) at risk for pulmonary hypoplasia to determine observed-expected lung volume ratio. This group was stratified according to pregnancy management, and observed-expected ratios were correlated with outcome. In fetuses with isolated congenital diaphragmatic hernia (CDH) (n = 19), observed-expected ratio was correlated with lung-head ratio, neonatal survival in pregnancies managed expectantly (n = 13), and/or lung-body weight ratio at necropsy (n = 9). For that purpose, linear regression correlation was used with the Pearson correlation coefficient; P < .05 was considered to indicate a significant difference. Results: Total fetal lung volume correlated best with total FBV (r = 0.96, P < .05). Observed-expected ratio based on FBV correlated with lung-head ratio in patients with CDH (r = 0.71, P < .001) and with lung-body weight ratio at necropsy (r = 0.68, P < .05) and could be used to help predict neonatal survival. Conclusion: FBV measured with MR imaging can be used as a single parameter in an algorithm and showed closest correlation with normal total fetal lung volume. In the transition from second to third trimester, this algorithm enabled calculation of the observed-expected ratio and prediction of outcome in fetuses at risk for pulmonary hypoplasia. (c) RSNA, 2006.