Birth Defects

Risk Factors

While approximately 10% of birth defects are attributed to environmental factors and 20% are attributed to single-gene or chromosomal defects that may be inherited or represent new mutations, the causes of the remaining 70% of birth defects remain unknown. Most birth defects are believed to arise from interactions between genes or interactions between environmental factors and genes.

Teratogens are agents that can cause birth defects. The timing of exposure to a teratogen during pregnancy is important: The greatest risk for structural malformations exists for exposures occurring between the third and eighth week of gestation (the embryonic period), when most organ systems are developing (organogenesis). The period in which an organ or system is at greatest risk of damage from a teratogen is referred to as the critical period: For some organs and systems, this period extends beyond the eighth week into the fetal period. However, structural birth defects are less likely to occur after the eighth week because most organ systems have already been established. Some well-known human teratogens include thalidomide (a sedative), isotretinoin (an antiacne medication sold under the brand name Accutane), valproic acid (an antiseizure medication sold under the brand name Depakote), warfarin (an anticoagulant sold under the brand name Coumadin), and mercury (a heavy metal).

Other factors have been identified as risk factors for birth defects, such as dietary deficiencies (vitamin A), maternal behaviors (smoking and alcohol [Page 99]consumption), maternal illnesses (diabetes mellitus and rubella infection), and family history of a similar defect (hypospadias). Some defects vary by maternal age (Down syndrome), race (tetralogy of Fallot), ethnicity (spina bifida), or infant sex (anencephaly). Epidemiologic research has demonstrated that taking a multivitamin containing folic acid, a B vitamin, before conception and in early pregnancy lowers the risk of having a baby with a neural tube defect. Other research suggests that folic acid may reduce the risk of other types of birth defects. Since the mandated folic acid fortification of cereal grains and flour in the United States in 1998, the occurrence of neural tube defects has decreased and reductions in other birth defects have been observed.

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