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california birth defects monitoring program

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Genetic and Environmental Factors

Genetic variations and environmental factors play key roles in the causes of birth defects. Twenty percent of birth defects are caused by chromosomes inherited from parents.  Children with birth defects may have too few or too many chromosomes, which then influences their structural development.

Environmental factors such as infection, exposure to hazardous waste and drug use by mothers are associated with approximately 10 percent of birth defects.  The occurrence of heart defects has been linked to infections such as rubella (German measles) and toxoplasmosis (a parasitic disease). Cleft palate has been associated with radiation exposure during the first trimester of pregnancy.

Although 70 percent of birth defects have an unknown causal factor, many birth defects are caused by a gene-environment interaction.  An unborn infant may carry  certain genes that would cause a birth defect in combination with harmful environmental factors. One example of gene-environment interaction was uncovered in a past study of smoking and oral clefts. A study conducted by CBDMP combined interview data detailing mothers' smoking habits with DNA testing to determine the infants' status for a particular gene, transforming growth factor alpha (TGFA). It was found that:

  • Mothers who smoked in early pregnancy were more likely to have babies with oral clefts. Risk increased with the amount smoked—heavy smokers had double the risk of nonsmokers.
  • Babies with a rare form of the TGFA gene were slightly more likely to have isolated cleft palate but not cleft lip. The combination of both factors—gene status and smoking—created a huge jump in risk. Babies with the uncommon gene form whose mothers smoked had a 6-10 times higher risk of oral clefts.
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