Some birth defects are caused by a single, well-documented factor, such as a missing or abnormal gene or chromosome. Many are caused by a combination of several factors. However, the causes of 70 percent of all birth defects are not known, according to the Centers for Disease Control and Prevention (CDC).

Many birth defects are the direct result of genetic disorders. These are often chromosomal conditions, such as an absent or extra chromosome. Even a single abnormal gene can cause a birth defect. In normal development, a baby receives half of its genes from the mother and half from the father. Genes may be dominant or recessive. For example, if one parent has the recessive gene for the enzyme associated with phenylketonuria (PKU) and the other parent has a dominant gene for the PKU enzyme, the baby will have functioning enzymes and will not develop PKU. Many genetic disorders such as PKU can occur only when both parents contribute a recessive gene, leading to expression of the defect.

Genetic birth defects can also result from a spontaneous mutation in a gene or during embryonic development. Some genes only increase an infant’s risk of a birth defect. These can be passed down for many generations before the defect actually occurs.

Other known causes of birth defects involve environmental factors affecting the fetus and the mother. These environmental factors may be divided into categories such as:

• Teratogens. Any harmful substances that the mother takes in and that pass through the placenta into the fetal bloodstream. Among the best known teratogens is alcohol, which causes fetal alcohol syndrome in about one out of every 1,000 infants, according to the March of Dimes. Maternal smoking has been linked to many birth defects, including cleft lip and palate, heart defects, clubfoot and other limb defects. Recreational drugs are also dangerous to the fetus. The March of Dimes reports that infants whose mothers used cocaine in early pregnancy are five times more likely to have urinary tract defects and damage to the central nervous system. Exposure to certain toxic chemicals, especially solvents (chemicals used in paints, varnishes, lacquers, adhesives, glues and degreasing/cleaning agents), during pregnancy also may increase a woman’s risk of miscarriage or having a baby with birth defects. In addition, a recent study found that men who are exposed to solvents on a regular basis may be at greater risk of fathering offspring with birth defects and low birth weight.
  
  
• Medications. Many medications including those used for treating blood pressure disorders, epilepsy and mental illnesses (e.g., depression) have also been linked to birth defects. However, while women are encouraged to avoid alcohol, nicotine and recreational drugs while pregnant, they may not be able to avoid all medications. All women who are or may be pregnant should discuss all of their medications, including over-the-counter medications, with their doctor.
  
  
• Infections and illnesses. Women with diabetes or high blood pressure have an increased risk of having a child with a birth defect, particularly if these conditions are not under control during the pregnancy. Infections during pregnancy may also lead to birth defects. Rubella carries the highest risk, although it is much less common now that the rubella vaccination is available. According to the March of Dimes, infants have a 25 percent chance of displaying at least one feature of congenital rubella syndrome if the mother is infected during the first trimester of pregnancy. Other infections that may result in birth defects include cytomegalovirus, toxoplasmosis and chickenpox. Sexually transmitted diseases (e.g., syphilis, genital herpes) may also lead to birth defects.
  
  
• Maternal nutrition. Infants born to women who are particularly stressed, obese, or malnourished also have an increased risk of birth defects, particularly neural tube defects. Many neural tube defects are linked to a deficiency in the B vitamin folic acid before and during pregnancy.

In addition to the known causes, some maternal risk factors increase the likelihood of birth defects. These include:

• Age. Infants born to teenagers and women over age 35 have a higher risk of birth defects, particularly chromosomal defects (e.g., Down syndrome). The father’s age may also play a role in the occurrence of birth defects. According to a new study, the genetic quality of sperm gradually deteriorates as men get older, increasing a man’s risk of infertility, fathering unsuccessful pregnancies (miscarriages) and passing along some genetic defects to his offspring (e.g., dwarfism). Another study also found that men who undergo surgery to reverse a vasectomy (procedure that permanently sterilizes a man and takes away his ability to impregnate a woman) may be at greater risk of having genetically defective sperm, which can increase the likelihood of birth defects in their offspring.
  
  
• Pregnancy history. Women with a history of miscarriage or who have had previous children with birth defects also have a higher risk of having another child with a birth defect.
  
  
• Premature birth. Infants born prematurely have a greater risk of having some birth defects, such as cerebral palsy.
  
  
• Multiple pregnancies. Infants in multiple pregnancies, especially higher order multiple pregnancies (three or more babies) are more likely to have birth defects.