Cardiac embyogenesis occurs in the first 6 to 7 weeks of human development. Although it is during this time that many of the major cardiovascular defects develop, many of these lesions continue to evolve and others develop in the latter half of gestation. There may be development or progression of ventricular inflow or outflow tract and arch obstruction, and ventricular or great artery hypoplasia. There may be progressive antrioventricular or semi-lunar valve regurgitation which can compromise the fetal circulation. There may be development of dysrhythmias, primary myocardial disease and heart failure. The fetal shunts, the foramen ovale and ductus arteriosus, may change in form and function. Finally, cardiac tumors may develop, grow, or regress. Knowledge of the mechanisms of and potential for progression in fetal heart disease is critical for counseling regarding prognosis and for planning of prenatal and neonatal management.