A newborn turns blue when crying; echo shows a hole between ventricles. This is a ventricular septal defect (VSD) — the interventricular septum failed to fuse fully during week 4–7. Oxygenated blood from the high-pressure left ventricle shunts into the right, flooding the lungs and starving the body — the baby tires during feeding and may look dusky. Small VSDs often close on their own; large ones need surgical patching. It is the commonest congenital heart defect.