UNLABELLED: There are conflicting results on the association between maternal vitamin D concentrations during pregnancy and respiratory outcomes for their offspring. However, published studies have mainly focused on the second and third trimesters of pregnancy or on high-risk population. The main objective of this study was to evaluate the association between vitamin D plasma concentrations in the first trimester (T1) of pregnancy and effective specific airway resistance (sR CONCLUSION: In a cohort of young children, airway resistance and asthma were not associated with early vitamin D deficiency during pregnancy. TRIAL REGISTRATION: This study is registered with the ClinicalTrials.gov identifier NCT03229317. WHAT IS KNOWN: • There are conflicting results on the association between maternal vitamin D concentrations during pregnancy and respiratory outcomes for their offspring. However, published studies have mainly focused on the second and third trimesters of pregnancy or on high-risk population. WHAT IS NEW: • In a cohort of 5- to 6-year-old children assessed prospectively, there was no significant difference in sReff or any other lung function measurements depending on maternal vitamin D status during the first trimester of pregnancy.