Polyploid cells and organisms are those containing more than two paired (homologous) sets of chromosomes. Most species whose cells have nuclei (Eukaryotes) are diploid, meaning they have two sets of chromosomes—one set inherited from each parent. However, polyploidy is found in some organisms and is especially common in plants. In addition, polyploidy occurs in some tissues of animals that are otherwise diploid, such as human muscle tissues. This is known as endopolyploidy. Species whose cells do not have nuclei, that is, Prokaryotes, may be polyploid organisms, as seen in the large bacterium "Epulopiscium fishelsoni" . Hence ploidy is defined with respect to a cell. Most eukaryotes have diploid somatic cells, but produce haploid gametes (eggs and sperm) by meiosis. A monoploid has only one set of chromosomes, and the term is usually only applied to cells or organisms that are normally diploid. Male bees and other Hymenoptera, for example, are monoploid. Unlike animals, plants and multicellular algae have life cycles with two alternating multicellular generations. The gametophyte generation is haploid, and produces gametes by mitosis, the sporophyte generation is diploid and produces spores by meiosis.

Polyploidy refers to a numerical change in a whole set of chromosomes. Organisms in which a particular chromosome, or chromosome segment, is under- or overrepresented are said to be aneuploid (from the Greek words meaning "not", "good", and "fold"). Therefore, the distinction between aneuploidy and polyploidy is that aneuploidy refers to a numerical change in part of the chromosome set, whereas polyploidy refers to a numerical change in the whole set of chromosomes.

Polyploidy may occur due to abnormal cell division, either during mitosis, or commonly during metaphase I in meiosis. In addition, it can be induced in plants and cell cultures by some chemicals: the best known is colchicine, which can result in chromosome doubling, though its use may have other less obvious consequences as well. Oryzalin will also double the existing chromosome content.

Polyploidy occurs in highly differentiated human tissues in the liver, heart muscle and bone marrow. It occurs in the somatic cells of some animals, such as goldfish, salmon, and salamanders, but is especially common among ferns and flowering plants (see "Hibiscus rosa-sinensis"), including both wild and cultivated species. Wheat, for example, after millennia of hybridization and modification by humans, has strains that are diploid (two sets of chromosomes), tetraploid (four sets of chromosomes) with the common name of durum or macaroni wheat, and hexaploid (six sets of chromosomes) with the common name of bread wheat. Many agriculturally important plants of the genus "Brassica" are also tetraploids.

Polyploidization is a mechanism of sympatric speciation because polyploids are usually unable to interbreed with their diploid ancestors. An example is the plant "Erythranthe peregrina". Sequencing confirmed that this species originated from "E. x robertsii", a sterile triploid hybrid between "E. guttata" and "E. lutea," both of which have been introduced and naturalised in the United Kingdom. New populations of "E. peregrina" arose on the Scottish mainland and the Orkney Islands via genome duplication from local populations of "E. x robertsii". Because of a rare genetic mutation, "E. peregrina" is not sterile.

Polyploid types are labeled according to the number of chromosome sets in the nucleus. The letter "x" is used to represent the number of chromosomes in a single set.

- triploid (three sets; 3"x"), for example seedless watermelons, common in the phylum Tardigrada

- tetraploid (four sets; 4"x"), for example Salmonidae fish, the cotton "Gossypium hirsutum "

- pentaploid (five sets; 5"x"), for example Kenai Birch ("Betula papyrifera" var. "kenaica")

- hexaploid (six sets; 6"x"), for example wheat, kiwifruit

- heptaploid or septaploid (seven sets; 7"x")

- octaploid or octoploid, (eight sets; 8"x"), for example "Acipenser" (genus of sturgeon fish), dahlias

- decaploid (ten sets; 10"x"), for example certain strawberries

- dodecaploid (twelve sets; 12"x"), for example the plants "Celosia argentea" and "Spartina anglica" or the amphibian "Xenopus ruwenzoriensis".

DFTD tumours are "large, solid, soft tissue masses usually with flattened, centrally ulcerated, and surfaces", which are "typically multicentric, appearing first in the oral, face, or neck regions", and are " to nodular aggregates of round to spindle-shaped cells, often within a pseudocapsule and divided into lobules by delicate fibrous septae". The disease is described as being aggressive, locally. Tumours can cover the eyes.

As Iain O'Neill notes, it also presents a "high rate of regional lymph node involvement and systemic metastasis"; of the avenues by which the cancer can spread systemically, metastasis to the lungs, spleen, and heart are observed, as is secondary "intracranial involvement". O'Neill also notes that "organ involvement and superimposed infection may... contribute to mortality." Moreover, growth of large tumours impedes feeding, and starvation is another cause of death in affected devils.

Devil facial tumour disease (DFTD) is an aggressive non-viral clonally transmissible cancer which affects Tasmanian devils, a marsupial native to Australia.

DFTD was first described in 1996. In the subsequent decade the disease ravaged Tasmania's wild devils, with estimates of decline ranging from 20% to as much as 50% of the devil population, across over 65% of the state. Affected high-density populations suffer up to 100% mortality in 12–18 months. The disease has mainly been concentrated in Tasmania's eastern half. Visible signs of DFTD begin with lesions and lumps around the mouth. These develop into cancerous tumours that may spread from the face to the entire body. Devils usually die within six months from organ failure, secondary infection, or metabolic starvation as the tumours interfere with feeding. As of 2010, 80% of population is infected, and only 0.1% is not affected. DFTD affects males and females equally. As of 2010, the population had been reduced by 70% (from 1996 census data), and if a cure is not found, a prediction has been made that the species will become extinct by 2035.

The most plausible route of transmission is through biting, particularly when canine teeth come into direct contact with the diseased cells. Other modes of transmission that cannot be discounted, yet haven't been conclusively proven, are the ingesting of an infected carcass and the sharing of food, both of which involve an allogeneic transfer of cells between unrelated individuals.. The cancer seems to infect the fittest devil individuals, which are socially dominant. Animals that eventually become infected survive at a higher rate and reproduce more before dying of the disease than devils that don’t get the cancer.

As of 2010, six females had been reported to have been found with partial immunity to DTF, and breeding in captivity was begun in an attempt to save the population.

Miscarriage, also known as spontaneous abortion and pregnancy loss, is the natural death of an embryo or fetus before it is able to survive independently. Some use the cutoff of 20 weeks of gestation, after which fetal death is known as a stillbirth. The most common symptom of a miscarriage is vaginal bleeding with or without pain. Sadness, anxiety and guilt often occur afterwards. Tissue and clot-like material may leave the uterus and pass through and out of the vagina. When a woman keeps having miscarriages, infertility is present.

Risk factors for miscarriage include an older parent, previous miscarriage, exposure to tobacco smoke, obesity, diabetes, thyroid problems, and drug or alcohol use. About 80% of miscarriages occur in the first 12 weeks of pregnancy (the first trimester). The underlying cause in about half of cases involves chromosomal abnormalities. Diagnosis of a miscarriage may involve checking to see if the cervix is open or closed, testing blood levels of human chorionic gonadotropin (hCG), and an ultrasound. Other conditions that can produce similar symptoms include an ectopic pregnancy and implantation bleeding.

Prevention is occasionally possible with good prenatal care. Avoiding drugs, alcohol, infectious diseases, and radiation may prevent miscarriage. No specific treatment is usually needed during the first 7 to 14 days. Most miscarriages will complete without additional interventions. Occasionally the medication misoprostol or a procedure such as vacuum aspiration is used to remove the remaining tissue. Women who have a blood type of rhesus negative (Rh negative) may require Rho(D) immune globulin. Pain medication may be beneficial. Emotional support may help with negative emotions.

Miscarriage is the most common complication of early pregnancy. Among women who know they are pregnant, the miscarriage rate is roughly 10% to 20%, while rates among all fertilisation is around 30% to 50%. In those under the age of 35 the risk is about 10% while it is about 45% in those over the age of 40. Risk begins to increase around the age of 30. About 5% of women have two miscarriages in a row. Some recommend not using the term "abortion" in discussions with those experiencing a miscarriage in an effort to decrease distress.

Signs of a miscarriage include vaginal spotting, abdominal pain or cramping, and fluid or tissue passing from the vagina. Bleeding can be a symptom of miscarriage, but many women also have bleeding in early pregnancy and don't miscarry. Bleeding during pregnancy may be referred to as a threatened miscarriage. Of those who seek clinical treatment for bleeding during pregnancy, about half will miscarry. Miscarriage may be detected during an ultrasound exam, or through serial human chorionic gonadotropin (HCG) testing.