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A fetus’ arms and hands form between the fourth and sixth week of pregnancy. Any disruption of this process can lead to a congenital hand difference. There are many factors that can affect the development of the human hand. These factors are generally divided into genetic and environmental.

Genetic factors involve changes to the information contained within the genes responsible for hand and arm formation. Genes are the basic biological unit of heredity and are passed on to children from their parents. They contain instructions for the growth and function of each cell in the body. In the case of hand differences, the genetic changes generally occur for no apparent reason (spontaneous). Less common are changes that run in families.

Environmental factors include infections and certain drugs, such as thalidomide (a drug used to treat nausea) and some drugs used for chemotherapy. These factors may cause a breakdown in otherwise healthy tissue, altering the developmental process and leading to a difference in hand formation.

Some hand differences can be explained by these factors, while others have no known cause. In some cases, the hand difference is an isolated event. In other cases, the difference is part of a syndrome that affects multiple parts of the body.

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