Birth defects are problems that occur in one or more parts of the body while a baby is developing in the mother’s body. Most birth defects happen during the first three months of pregnancy. Birth defects can affect almost any part of the body and may affect how the body looks or works, or both. Birth defects can be mild to severe. Some are structural problems that are easy to see. While doctors might need to use special tests to find others. The well-being of each child with a birth defect depends mostly on which organ or body part is involved and how much it’s affected. In the United States, about three babies in 100 are born with a birth defect.