< Back to Previous Page Common Types of Birth Defects Metabolic or structural birth defects are the most common type of birth defects and they occur when certain body parts malformed or completely absent at birth.  These birth defects usually happen when there is an imbalance of body chemistry during the development of the fetus.  It is estimated that one out of every 33 births will suffer from some sort of structural birth defect.  The most frequent structural birth defects in infants are a variety of heart defects that average about once in every 100 births in the US alone. Other common structural birth defects include: Missing limbs Spina bifida Mental retardation Hypospadias Metabolic birth defects occur in one out of 3,500 births and are frequently due to missing or malformed enzymes in living cells.  Enzymes are vital protein essential for processing chemicals in the body while the fetus is forming.  Metabolic birth defects can affect a child for the rest of their life and can even cause death for a child but metabolic birth defects are not always clearly visible from a physical perspective. Common types of metabolic birth defects include: Hearing loss PKU (phenylketonuria) Hypothyroidism Tay-Sachs disease Galactosemia Sickle Cell Anemia Congenital adrenal hyperplasia (CAH) Birth defects can also occur if the mother suffers from certain congenital infections during pregnancy.  Congenital infections do not always result in birth defects but the risks are higher to the unborn child if the pregnant mother suffers from one. The congenital infections most likely to increase the risk of birth defects include: Rubella (German measles) Syphilis Toxoplasmosis Parvovirus Cytomegalovirus (CMV) Chickenpox