Small for gestational age is associated with reduced lung function in middle age: A prospective study from first to fifth decade of life

Background and Objective The association between birth weight, particularly relative to gestational age, and adult lung function is uncertain. We investigated the associations between birth weight relative to gestational age and measures of lung function in middle age, and mediation of these associations by adult height. Methods Participants in the Tasmanian Longitudinal Health Study who had both known birth weight and lung function assessment at age 45 years were included (n = 849). Linear regression models were fitted to investigate the association between small for gestational age and birth weight with post-bronchodilator lung function measures (forced expiratory volume in 1 second [FEV1], forced vital capacity [FVC], FEV1/FVC, diffusing capacity for carbon monoxide [D(L)co], residual volume [RV] and total lung capacity [TLC]), adjusting for potential confounders. The contribution of adult height as a mediator of these associations was investigated. Results Compared with infants born with normal weight for gestational age, those born small for gestational age had reduced FEV1 (coefficient: -191 ml [95%CI: -296, -87]), FVC (-205 ml [-330, -81]), TLC (-292 ml [-492, -92]), RV (-126 ml [-253, 0]) and D(L)co (-0.42 mmol/min/kPa [-0.79, -0.041]) at age 45 years. However, they had comparable FEV1/FVC. For every 1 kg increase in birth weight, lung function indices increased by an average of 117 ml (95%CI: 40, 196) for FEV1, 124 ml (30, 218) for FVC, 215 ml (66, 365) for TLC and 0.36 mmol/min/kPa (0.11, 0.62) for D(L)co, independent of gestational age, but again not for FEV1/FVC. These associations were significantly mediated by adult height (56%-90%). Conclusion Small for gestational age was associated with reduced lung function that is likely due to smaller lungs with little evidence of any specific parenchymal impairment.