Before birth, the mother’s body provides for all the baby’s needs, regardless of the baby’s anatomy. Nutrients and oxygen are carried across the placenta to the foetus, and carbon dioxide and other waste products are removed. As the foetus does not need to breathe before birth, there are short-circuits in the heart allowing foetal blood to bypass the inactive lungs.
Birth separates the baby from the placenta, so the mother’s circulation can no longer bring oxygen or remove carbon dioxide, and the baby’s own lungs must now do this.
The temporary short-circuits needed before birth must close, otherwise the baby’s blood will continue to bypass the lungs, and the baby will be short of oxygen. If a shortcircuit or detour persists after birth, this is considered a form of CHD, even if the detour is situated well away from the heart, for example connecting major blood vessels.
(The Heart and Stroke Foundation SA, Health24, updated January 2008)