Congenital anomalies in conjunction with intrauterine growth restriction account for 50-60% of fetal mortality. This article describes major birth defects by systems; most can be diagnosed by ultrasound while others cannot. Some anomalies develop later in pregnancy and in some cases sonographic examination may suggest the presence of a nonexistent abnormality. Incidence, etiology and risk factors of congenital malformations are described, as well as the importance and characteristics of prenatal diagnosis and prevention through folic acid supplementation in the diet.