The cause of 40-60% of congenital anomalies in humans is unknown. These are referred to as sporadic, a term that implies an unknown cause, random occurrence regardless of maternal living conditions, and a low recurrence risk for future children. For 20-25% of anomalies there seems to be a”multifactorial” cause, meaning a complex interaction of multiple minor genetic anomalies with environmental risk factors.
Another 10-13% of anomalies have a purely environmental cause (e.g. Infections, illness,or drug abuse in the mother). Only 12-25%of anomalies have a purely genetic cause. Of these, the majority are chromosomal anomalies. Genetic causes of congenital anomalies include inheritance of abnormal genes from the parents, as well as new mutations in one of the germ cells that gave rise to the fetus.
Environmental causes of congenital anomalies are referred to as teratogenic. These are generally problems with the mother’s environment.Teratogens can include dietary deficiencies, toxins, or infections. For example, dietary deficiency of maternal folic acid is associated with spina bifida.
Ingestion of harmful substances by the mother (e.g.,alcohol, mercury, or prescription drugs such as phenytoin) can cause recognizable combinations of birth defects. Several infections which a mother can contract during pregnancy can also be teratogenic. These are referred to as the torch infections.
The greatest risk of a malformation due to environmental exposure to a teratogen (terato = monster, gen = producing) between the third and eighth week of gestation. Before this time, any damage to the embryo is likely to result in fatality, and the baby will not be born. After eight weeks, the fetus and its organs are more developed and less sensitive to terotogenic incidents.
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