Polymelia is a birth defect in which an affected individual has more than the usual number of limbs. It is a type of dysmelia. In humans and most land-dwelling vertebrates, this means having five or more limbs. The extra limb is most commonly shrunken and/or deformed.

The prevalence of polymelia alone in the Indian population is difficult to ascertain; however, the prevalence of birth defects as a whole is 61 to 69.9/1000 live births. Since polymelia is an extremely rare condition, it is plausible that it forms only a minute fraction of the total prevalence of birth defects. Polymelia can be caused by one of several factors:

Genetic Factors: These include defects in the chromosomes and transgenes. One or more genes may undergo a change, technically termed a mutation. Sometimes, a part of a gene may be missing. These changes can result in improper cell division and/or proliferation, leading to polymelia. In this genetic birth disorder, conjoined twins can sometimes be formed in which one of them undergoes degeneration while still in the womb, but its shrunken limbs may remain attached to the remaining fetus, thereby giving rise to polymelia.

External Factors: Exposure to infectious agents, medications, and chemicals during pregnancy can influence the pattern of gene expression that can lead to polymelia. The thalidomide disaster is a classic example, where pregnant women consumed a cough syrup containing the drug thalidomide.

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