Superfecundation
Superfecundation is the fertilization of two or more ova from the same cycle by sperm from separate acts of sexual intercourse.  The term is also sometimes used to refer to the instances of two different males fathering fraternal twins, though this is more accurately known as heteropaternal superfecundation.  This therefore leads to the possibility of twins also being half-siblings. The term superfecundation is derived from fecund, meaning the ability to produce offspring.

# Conception
Superfecundation most commonly happens within hours or days of the first instance of fertilization with ova released during the same cycle. There is a small time window when eggs are able to be fertilized. Sperm cells can live inside a womans body for 4-5 days. Once ovulation occurs, the egg remains viable for 12-48 hours before it begins to disintegrate. Thus, the fertile period can span 5-7 days. Ovulation is usually suspended during pregnancy to prevent further ova becoming fertilised and to help increase the chances of a full term pregnancy.  However, if an ovum is released after the female was already impregnated when previously ovulating, there is a chance of a second pregnancy -- albeit at a different stage of development.  This is known as superfetation.

# Heteropaternal superfecundation
Heteropaternal superfecundation is very rare in humans, though more common in other animals (such as cats).  This is primarily due to the predominance of 1:1 heterosexual partner pairings among humans.  A woman who has sexual relations with two or more men at short intervals within the same ovulatory period can be impregnated by both men.
There have been noted examples in the past. The first recorded case was made by John Archer, an American physician  in 1810 and is discussed in Williams Obstetrics (1980). According to Archer, a white woman who had sex with a black man and a white man within a short time subsequently gave birth to twinsone white, one mulatto. Other cases have been reported since.  This is similar to some incidents of mixed twins.

# Use in mythology
In Greek Mythology, Heracles and his twin, Iphicles, are examples of heteropaternal superfecundation, one from the god Zeus and the other from a mortal man.
In Greek Mythology, Leda conceives four children (Castor, Pollux, Helen and Clytemnestra) in the same night by two different men. Two are children of Zeus, who conceived disguised as a swan, while the other two were children of Leda's mortal husband, Tyndareus. Which men father which children varies widely among accounts. The heteropaternal superfecundation involved in this myth is especially unusual, because Leda did not give birth to the children, but laid eggs that hatched them, (because Zeus was in the form of a swan when he seduced her). This story would mean that Castor and Pollux the Gemini twins were born of different fathers.

# Literature
In John Steinbeck's novel East of Eden, the twin brothers Caleb and Aaron have different fathers.

# Television
- On the NBC soap opera Days of our lives there is a current storyline where a character named Sami Brady had fraternal twins born of two different fathers. She had been blackmailed into having sex with one man hours after having sex with her husband. It is unclear if the story of the twins having different fathers will stand or will be revealed to be false some time in the future.
- A similar story took place on One Life To Live when it was revealed that years ago show matriarch Victoria Buchanan gave birth to twin girls Natalie and Jessica Buchanan born from different fathers and separated at birth. Again one of the fathers raped the mother.
- A similar storyline occurred on Guiding Light. Blake discovered that one of her twins was fathered by Rick and the other by Ross. Eventually it was revealed that they were really both Ross's children.