Birth Weight and Diaphragmatic Ultrasound Measurements During the First 48 Hours in Term Neonates: A Secondary Longitudinal Analysis

Background/Objectives: Neonatal diaphragm ultrasound measurements may vary with body size. We examined associations between birth weight and repeated bilateral diaphragm measurements during the first 48 h after birth. Methods: This exploratory secondary post hoc analysis included 116 term neonates assessed at approximately 1, 24, and 48 h (348 examinations). Associations of birth weight with diaphragm thickness, diaphragmatic thickening fraction (DTF), and excursion were estimated using generalized estimating equations adjusted for gestational age, sex, and assessment time. Mean bilateral end-expiratory thickness was a derived post hoc summary, and absolute inspiratory thickening (ΔT) was examined additionally. Results: Across the 12 exploratory birth-weight outcome tests, none remained below 0.05 after global Holm correction, and this global result is the primary summary of this analysis. The post hoc mean bilateral end-expiratory thickness summary showed a nominal positive association with birth weight (β = 0.0876 mm/kg; 95% CI, 0.0112–0.1640; nominal p = 0.0246; global Holm-adjusted p = 0.2465); this is reported as a descriptive, hypothesis-generating observation and not as the main finding. All four side-specific thickness coefficients were nominally positive, but none remained below 0.05 after global adjustment. Birth weight showed little evidence of association with excursion or ΔT. Right-sided DTF showed an inverse nominal association (β = −1.4779 percentage points/kg; 95% CI, −2.5933 to −0.3624; nominal p = 0.0094; global Holm-adjusted p = 0.1130); given unresolved DTF provenance uncertainty (see Methods), this estimate is not used to support any physiological conclusion. Conclusions: None of the 12 exploratory birth-weight associations remained below 0.05 after global Holm correction, and this negative global result, rather than any single nominal association, is the principal finding of this analysis. The post hoc bilateral thickness association is reported descriptively and should not be regarded as a main finding; ΔT and DTF are reported as auxiliary post hoc observations. These findings are incremental and hypothesis-generating and require prospective, prespecified validation.