Best cardiologist near Yelahanka Bangalore Causes of Congenital heart defects Congenital heart defects usually develop while a baby is in the womb.Heart defects can develop as the heart develops, about a month after conception, changing the flow of blood in the heart. Some medical conditions, medications and genes may play a role in causing heart defects. Heart defects can also develop in adults. As you age, your heart's structure can change, causing a heart defect.