Congenital heart defects (congenital heart disease) , refers to certain abnormalities in the structure of the heart or in the blood vessels around the heart. The heart of an infant in the womb normally begins to develop in the 5th week of pregnancy, and will be fully formed by around the 12th week. Genetic material known as genes is responsible for the construction of the infant's heart. Genes direct the action, order, and structure of the heart, and act as a blueprint for the rest of the body. A huge number of genes are required for the development of an infant's heart during pregnancy, and each of these genes must fit together correctly. If there is a defect in any one gene, or in any number of genes, then it may cause structural abnormalities to develop in the heart of an unborn child. At present, only a few defects in a small number of genes have been scientifically proven to cause congenital heart defects, and for the majority of patients, the doctor will be unable to identify the exact cause of the condition. The doctor will be aware, however, of certain factors which increase the risk of congenital heart defects in unborn children, by monitoring for such factors during pregnancy as the infant's heart develops.