What causes babies to be born with heart disease?

We do not know what causes the very slight natural tendency. What is
now well-known is that if a mother catches German measles within the first
three months of her pregnancy, that is, at a time when the heart and other
organs are being formed, there is a considerable risk of the baby having
heart disease and other defects. So if a mother early in pregnancy is in
contact with German measles, she should consult her doctor immediately, for
steps can be taken to prevent damage occurring. This cause will shortly
become less important as vaccination against German measles for woman
who have not had it, is now available.
Drugs like thalidomide taken in the first three months of pregnancy
have been known to cause heart disease, but laws covering the production
and use of drugs should prevent this happening again.
Though it is possible that viruses other than German measles may
cause congenital heart disease, there is no proof at present. Most of the
causes are unknown and appear to be sheer bad luck, due to a slight fault in
the complex growth of the heart before birth.