Congenital heart diseases (CHDs) are the most common type of birth anomaly and a major source of childhood death and illness. Studies that have included both mothers and children have shown that CHD in the child is more likely to occur in those with mothers who are older, smoke during pregnancy, weigh more and have other pregnancy problems such as increased blood pressure and blood sugar levels.

Few studies have explored the extent to which these associations are causal for CHDs. To our knowledge there are no large-scale studies, with healthy control groups that have used new analysis methods to explore whether these associations are causal.