Polymelia (from Greek πολυ- = "many" plus μέλος (plural μέλεα) = "limb") also known as "hydra syndrome" is a birth defect involving limbs (a type of dysmelia), in which the affected individual has more than the usual number of limbs. In humans and most land-dwelling animals, this means having five or more limbs. The extra limb is most commonly shrunken and/or deformed.

Sometimes an embryo started as conjoined twins, but one twin degenerated completely except for one or more limbs, which end up attached to the other twin.

Sometimes small extra legs between the normal legs are caused by the body axis forking in the dipygus condition.

Supernumerary body parts are most commonly a congenital disorder involving the growth of an additional part of the body and a deviation from the body plan. Body parts may be easily visible or hidden away, such as internal organs.

Many additional body parts form by the same process as conjoined twins: the zygote begins to split but fails to completely separate. This condition may also be a symptom of repeated occurrences of continuous inbreeding in a genetic line.

Vestigial structures are anatomical structures of organisms in a species which are considered to have lost much or all of their original function through evolution. These body parts can be classed as additional to the required functioning of the body. In human anatomy the vermiform appendix is sometimes classed as a vestigial remnant.

Prosthesis is an artificial extension that replaces a body part, and cybernetics is the study of computer technology in relation to organisms which can include replacement or additional body parts.

Body integrity identity disorder (BIID) is a psychiatric disorder in which a person thinks that they have one or more additional limbs than they should, despite having two arms and two legs. People with this condition often wish to amputate what they see as additional body parts.

A phantom limb is the sensation that a missing limb is still attached to the body.

A supernumerary phantom limb is the sensation of having an extra limb or body part despite no such limb actually existing. It is an uncommon syndrome, usually due to some kind of brain injuries in the somatosensory cortex or in some parts of the right hemisphere of the brain, usually due to a stroke in the brain.

A chimera is an animal or plant that has two or more different populations of genetically distinct cells that originated in different zygotes that have merged; anatomical structures are typically mixed depending on which cells are prevalent in different body parts, for example plants can have two different types of flowers.

A mosaic is a genetic anomaly similar in nature and effects to a chimera: genetically different populations of cells within one organism, originated from some propagated mutation of a single cell rather than from outside sources.

Dysmelia (from Gr. δυσ- "dys", "bad" + μέλ|ος "mél|os", "limb" + Eng. suff. -ia) is a congenital disorder of a limb resulting from a disturbance in embryonic development.

Dysmelia can be caused by

- inheritance of abnormal genes, e.g. polydactyly, ectrodactyly or brachydactyly, symptoms of deformed limbs then often occur in combination with other symptoms (syndromes)

- external causes during pregnancy (thus not inherited), e.g. via amniotic band syndrome

- teratogenic drugs (e.g. thalidomide, which causes phocomelia) or environmental chemicals

- ionizing radiation (nuclear weapons, radioiodine, radiation therapy)

- infections

- metabolic imbalance

Many mythological creatures like dragons, winged horses, and griffins have six limbs: four legs and two wings. The dragon's science is discussed in "Dragons: A Fantasy Made Real".

In Greek Mythology, the Hekatonkheires were said to each have one hundred hands. The Gegenees were a race of giants with six arms.

Sleipnir, Odin's horse in Norse mythology, has eight normal horse legs, and is usually depicted with limbs twinned at the shoulder or hip.

Several Hindu deities are depicted with multiple arms and sometimes also multiple legs.

Probably, the most well-known teratogenic drug is thalidomide. It was developed near the end of the 1950s by Chemie Grűnenthal as a sleep inducing aid and antiemetic. Because of its ability to prevent nausea it was prescribed for pregnant women in almost 50 countries worldwide between 1956–1962. Until William McBride published the study leading to its withdrawal from the market at 1961, about 8- 10 000 severely malformed children were born. The most typical disorder induced by thalidomide were reductional deformities of the long bones of the extremities. Phocomelia otherwise a rare deformity, which therefore helped to recognise the teratogenic effect of the new drug. Among other malformations caused by thalidomide were those of ears, eyes, brain, kidney, heart, digestive and respiratory tract. 40% of the prenatally affected children died soon after birth. As thalidomide is used today as a treatment for multiple myeloma and leprosy, several births of affected children were described in spite of the strictly required use of contraception among female patients treated by it.

Vitamin A, or retinol, is the sole vitamin which is embryotoxic even in a therapeutic dose, for example in multivitamins, because its metabolite, the retinoic acid, plays an important role as a signal molecule in the development of several tisues and organs. Its natural precursor, the β-carotene, is considered safe, whereas the consumption of animal liver can lead to malformation, as the liver stores lipophile vitamins, including retinol. Isotretinoin (13-cis-retinoic-acid; brand name Roaccutane), vitamine A analog, which is often used to treat severe acne, is such a strong teratogen that just a single dose taken by a pregnant woman (even transdermally) may result in serious birth defects. Because of this effect, most countries have systems in place to ensure that it is not given to pregnant women, and that the patient is aware of how important it is to prevent pregnancy during and at least one month after treatment. Medical guidelines also suggest that pregnant women should limit vitamin A intake to about 700 μg/day, as it has teratogenic potential when consumed in excess. Vitamine A and similar substances can induce spontaneous abortions, premature births, defects of eyes (microphthalmia), ears, thymus, face deformities, neurological (hydrocephalus, microcephalia) and cardiovascular defects, as well as mental retardation.

Tetracycline, an antibiotic, should never be prescribed to women in the reproductive age or children, because of its negative impact on bone mineralization and teeth mineralization. The "tetracycline teeth" have brown or grey colour as a result of a defective development of both the dentine and the enamel of teeth.

Several anticonvulsants are known to be highly teratogenic. Phenytoin, also known as diphenylhydantoin, along with carbamazepine is responsible for the fetal hydantoin syndrome, which may typically include broad nose base, cleft lip and/or palate, microcephalia, nails and fingers hypoplasia, intrauterine growth restriction and mental retardation. Trimethadione taken during pregnancy is responsible for the fetal trimethadione syndrome, characterized by craniofacial, cardiovascular, renal and spine malformations, along with a delay in mental and physical development. Valproate has anti-folate effects, leading to neural tube closure-related defects such as spina bifida. Lower IQ and autism have recently also been reported as a result of intrauterine valproate exposure.

Hormonal contraception is considered as harmless for the embryo. Peterka and Novotná do however state that syntethic progestines used to prevent miscarriage in the past frequently caused masculinization of the outer reproductive organs of female newborns due to their androgenic activity. Diethylstilbestrol is a synthetic estrogen used from the 1940s to 1971 when the prenatal exposition has been linked to the clear-cell adenocarcinoma of the vagina. Following studies showed elevated risks for other tumors and congenital malformations of the sex organs for both sexes.

All cytostatics are strong teratogens, abortion is usually recommended when pregnancy is found during or before chemotherapy. Aminopterin, a cytostatic drug with anti-folate effect, was used during the 1950s and 1960s to induce therapeutic abortions. In some cases the abortion didn´t happen, but the newborns suffered a fetal aminopterin syndrome consisting of growth retardation, craniosynostosis, hydrocephalus, facial dismorphities, mental retardation and/or leg defomities

For the survivors of the atomic bombing of Hiroshima and Nagasaki, who are known as the "Hibakusha", no statistically demonstrable increase of birth defects/congenital malformations was found among their later conceived children, or found in the later conceived children of cancer survivors who had previously received radiotherapy.

The surviving women of Hiroshima and Nagasaki who were able to conceive, though exposed to substantial amounts of radiation, later had children with no higher incidence of abnormalities/birth defects than in the Japanese population as a whole.

Relatively few studies have researched the effects of paternal radiation exposure on offspring. Following the Chernobyl disaster, it was assumed in the 1990s that the germ line of irradiated fathers suffered minisatellite mutations in the DNA, which was inherited by descendants. more recently however, the World Health Organization states, "children conceived before or after their father's exposure showed no statistically significant differences in mutation frequencies". This statistically insignificant increase was also seen by independent researchers analyzing the children of the liquidators. Animal studies have shown that incomparably "massive" doses of X-ray irradiation of male mice resulted in birth defects of the offspring.

In the 1980s, a relatively high prevalence of pediatric leukemia cases in children living near a nuclear processing plant in West Cumbria, UK, led researchers to investigate whether the cancer was a result of paternal radiation exposure. A significant association between paternal irradiation and offspring cancer was found, but further research areas close to other nuclear processing plants did not produce the same results. Later this was determined to be the Seascale cluster in which the leading hypothesis is the influx of foreign workers, who have a different rate of leukemia within their race than the British average, resulted in the observed cluster of 6 children more than expected around Cumbria.