The observed vs. expected lung-to-head ratio does not correct for the effect of gestational age on the lung-to-head ratio

Objectives: To assess whether the observed vs. expected lung-to-head ratio (o/e LHR) corrects for the dependence of the LHR on gestational age. Study design: Published data on right lung area (LA) and LHR were used to plot the 50th percentile and different fixed values of the o/e LHR (e.g. 30%) against gestational age from 16-32 weeks. The Z-scores for various fixed o/e LHR values and similar percent value of LA were calculated. The effect of using a fixed LHR or a fixed o/e LHR was tested against gestational age. The o/e LHR-equivalent to a fixed LHR of 1.0 was assessed against gestational age. Results: The LHR and the o/e LHR both increase with gestational age. The Z-score of a given fixed value of the o/e LHR (e.g. 30%) is not similar to the Z-score of the same percent (e.g., 30%) of the expected LA, and thus identifies different proportions of subjects. A fixed o/e LHR (e.g. 30%) results in different populations, depending on the gestational age. The o/e LHR equivalent to an LHR value of 1.0 decreases from 80% at 16 weeks to 30% at 32 weeks. Conclusions: The o/e LHR is not independent of gestational age. Studies using this parameter should be interpreted with caution.