A birth defect is an anomaly that is congenital, or present from birth. Birth defects are the leading cause of infant mortality, causing 22 percent of all infant deaths. Approximately 3 to 4 percent of all live births are affected by a birth defect; the causes of most of them are unknown. Some birth defects are considered to be physical, while others are thought of as functional. Physical birth defects result in the malformation of a physical organ or limb, whereas functional birth defects are those that cause primarily functional, rather than physical, problems. Functional birth defects include mental retardation, congenital hearing loss, early-onset vision impairment, and numerous other health concerns. They may be caused by single gene mutations, or they may be due to polygenic or multifactorial inheritance.

Birth defects may be found in isolation or they may occur in combination in one child, as part of a...