Congenital heart disease (CHD) is a group of diseases with defects in the heart that occur during fetal life when the heart is formed. CHD occurs in 1% of births each year with causes not fully known. Most cases are believed to be caused by effects of both genes and risk factors that could be changed to prevent CHD (1,2). We are interested in identifying these modifiable risk factors. Several risk factors have been suggested including maternal diet intake (3). We would like to investigate the possible association between maternal vitamin D status and the risk of CHD in the child. A study from 2018 found that low maternal vitamin D was associated with a higher risk of CHD in the offspring (2). That study used blood markers of vitamin D measured approx. 15-months after birth as a proxy for mother’s vitamin D in early pregnancy. Because of the strong effects of sunlight and season on vitamin D levels the measures might not be a good indicator of the mother’s vitamin D in early pregnancy. We would like to use ALSPAC data to investigate the association of mother’s blood vitamin D in early pregnancy on their child’s risk of CHD. ALSPAC is an ideal study to improving our understanding of whether ensuring pregnant women have sufficient vitamin D could prevent CHD. It has measures of blood vitamin D (4), details of all cases of CHD including those that were diagnosed after birth (5), and information on factors that could confound the results.