A birth defect is any disabling or fatal abnormality in structure, function or metabolism that is present at birth. The disability may be physical or mental. The medical community has identified several thousand different birth defects. Some are very minor, while others are severe or life-threatening.

Birth defects are the most common cause of infant death during the first year of life. According to the Centers for Disease Control and Prevention (CDC), more than 20 percent of all infant deaths occur due to birth defects. Those infants with moderate to severe birth defects who survive are more likely to be sick or have long-term problems than other infants. However, proper treatment often allows many children born with birth defects to lead full, healthy lives.

Pregnancy is an incredibly complex process. A single fertilized egg grows and divides into billions of cells and many different body systems, eventually forming a baby. Any change that affects that process, to any cell, gene or enzyme, can result in an anomaly. Many anomalies that occur during pregnancy are severe enough that they are incompatible with life and a miscarriage occurs. In other cases, the baby continues to develop but is born with a birth defect.

A birth defect can develop at any stage of pregnancy. However, most birth defects occur during the first three months of pregnancy, which is the most important stage of development due to the formation of the organs (e.g., the brain, spinal cord). Any developmental abnormalities that occur during these early stages of development can cause the fetus to grow defectively for the rest of the pregnancy. Any factor that interrupts the proper formation of the organs can result in a birth defect.