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Background Infants of asthmatic mothers have reduced lung function in early life for reasons that remain to be defined. The association between inhaled corticosteroid (ICS) use during pregnancy and lung function in the offspring has not been investigated.Objectives To investigate the association between ICS use during pregnancy and infant lung function.Methods Multivariable regression analysis of infant lung function at 4–6 weeks (tidal breathing flow volume loops and functional residual capacity (FRC)) associated with use of ICS during pregnancy.Results Among infants born to asthmatic mothers, the ratio of time to peak tidal expiratory flow to expiratory time (tPTEF:tE), corrected for end-expiratory lung volume (FRC), was improved in offsprings whose mothers used ICS during pregnancy compared with those who did not (n=161 ICS use vs n=25 no ICS use; coefficient 0.06 /mL, 95% CI 0.01 to 0.11, p=0.014). Compared with a control group of infants born to non-asthmatic mothers, there was a lower tPTEF:tE to FRC ratio in infants born to asthmatic mothers without ICS use (n=46 no asthma vs n=25 asthma no ICS use; coefficient −0.08 /mL, 95% CI −0.01 to −0.02, p=0.012) but not in infants born to asthmatic mothers with ICS use (n=46 no asthma vs n=161 asthma ICS use; coefficient −0.02 /mL, 95% CI −0.06 to 0.03, p=0.453).Conclusions The association between maternal asthma and impaired infant lung function diminished in infants whose mothers used ICS during pregnancy.Trial registration Australian New Zealand Clinical Trials Registry ACTRN12613000202763.