In this study we aim to analyse whether or not vitamin D levels during pregnancy influence lung development and, consequently, the occurrence of asthma and lung function performance in childhood and adolescence. To analyse these associations, we will use two methodologies. First, we will look at whether or not vitamin D levels measured in blood during pregnancy are associated with the onset of asthma and lung function parameters at 8-9 years and 15-16 years in the ALSPAC cohort. Since this association may be influenced by many confounding factors such as lifestyle, socio-economic status, the season of the year when the blood was obtained, we will use Mendelian Randomisation to overcome this limitation. We will analyse whether or not genetic variants that predispose to having low levels of vitamin D are associated with a greater predisposition to having asthma and poorer lung function in childhood and adolescence. We will use data from 4 different European cohorts and one Australian cohort and, after doing all the analysis separately, we will collect the results and do a meta-analysis using all data.