Twin in the context of "Look-alike"

Pregnancy is the time during which one or more offspring gestates inside a woman's uterus. A multiple pregnancy involves more than one offspring, such as with twins.

Conception usually occurs following vaginal intercourse, but can also occur through assisted reproductive technology procedures. A pregnancy may end in a live birth, a miscarriage, an induced abortion, or a stillbirth. Childbirth typically occurs around 40 weeks from the start of the last menstrual period (LMP), a span known as the gestational age; this is just over nine months. Counting by fertilization age, the length is about 38 weeks. Implantation occurs on average 8–9 days after fertilization. An embryo is the term for the developing offspring during the first seven weeks following implantation (i.e. ten weeks' gestational age), after which the term fetus is used until the birth of a baby.

Castor and Pollux (or Polydeuces) are twin half-brothers in Greek and Roman mythology, known together as the Dioscuri or Dioskouroi.

Their mother was Leda, but they had different fathers; Castor was the mortal son of Tyndareus, the king of Sparta, while Pollux was the divine son of Zeus, who seduced Leda in the guise of a swan. The pair are thus an example of heteropaternal superfecundation. Though accounts of their birth are varied, they are sometimes said to have been born from an egg, along with their twin sisters Helen of Troy and Clytemnestra.

A multiple birth is the culmination of a multiple pregnancy, wherein the mother gives birth to two or more babies. A term most applicable to vertebrate species, multiple births occur in most kinds of mammals, with varying frequencies. Such births are often named according to the number of offspring, as in twins and triplets. In non-humans, the whole group may also be referred to as a litter, and multiple births may be more common than single births. Multiple births in humans are the exception and can be exceptionally rare in the largest mammals.

A multiple pregnancy may be the result of the fertilization of a single egg that then splits to create identical fetuses, or it may be the result of the fertilization of multiple eggs that create fraternal ("non-identical") fetuses, or it may be a combination of these factors. A multiple pregnancy from a single zygote is called monozygotic, from two zygotes is called dizygotic, or from three or more zygotes is called polyzygotic. Similarly, the siblings themselves from a multiple birth may be referred to as monozygotic if they are identical or as dizygotic (in cases of twins) or polyzygotic (for three or more siblings) if they are fraternal, i.e., non-identical.

Superfecundation is the fertilization of two or more ova from the same menstrual cycle by sperm from the same or different males, whether through separate acts of intercourse or during a single sexual encounter with multiple males. This can potentially result in twin babies that have different biological fathers.

The term superfecundation is derived from fecund, meaning able to produce offspring. Homopaternal superfecundation is a form of twinning where fertilization of two separate ova occurs as a result of two or more distinct instances of intercourse or insemination with the same male partner or donor, leading to fraternal twins. Heteropaternal superfecundation, on the other hand, is an atypical form of twinning that results in twins that are genetically half siblings – sharing the same biological mother, but with different biological fathers.

Conjoined twins, popularly referred to as Siamese twins, are twins joined in utero. It is a very rare phenomenon, estimated to occur in anywhere between one in 50,000 births to one in 200,000 births, with a somewhat higher incidence in southwest Asia and Africa. Approximately half are stillborn, and an additional one-third die within 24 hours. Most live births are female, with a ratio of 3:1.

Two possible explanations of the cause of conjoined twins have been proposed. The one that is generally accepted is fission, in which the fertilized egg splits partially. The other explanation, no longer believed to be accurate, is fusion, in which the fertilized egg completely separates, but stem cells (that search for similar cells) find similar stem cells on the other twin and fuse the twins together. Conjoined twins and some monozygotic, but not conjoined, twins share a single common chorion, placenta, and amniotic sac in utero.