OBJECTIVE: The Observed to Expected Total Fetal Lung Volume (O/E-TFLV) ratio, calculated using fetal MRI, is a common method to assess pulmonary hypoplasia severity in congenital diaphragmatic hernia (CDH). However, its accuracy may be affected by uncertain gestational age (GA), inaccurate pregnancy dating, or abnormal fetal growth. This study aimed to evaluate whether GA determined by first-trimester dating or fetal size at imaging affects the ability of O/E-TFLV to predict 6-month survival in fetuses with isolated left-sided CDH. METHODS: Retrospective cohort study of fetuses with isolated left-sided CDH. O/E-TFLV was calculated using GA based on CRL (O/E-TFLV RESULTS: Ninety-seven fetuses underwent third-trimester MRI, with seventy-nine (81 %) also having second-trimester MRI. At six months, 80/97 (82.5 %) were alive. No significant differences were observed between O/E-TFLV CONCLUSION: There were no difference in the prediction of survival in isolated left-sided CDH fetuses using O/E-TFLV regardless of the method used to calculate O/E-TFLV.