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.
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 woman’s 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 fertilized 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 is very rare in humans, though more common in other animals (such as cats), and is extremely common in dogs, with stray dogs often whelping litters where every puppy is of a different father. 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 twins—one white, one of mixed-race. This is similar to some incidents of mixed twins. Other cases have been reported since, such as that of a Texas woman who gave birth to fraternal twins fathered by different men in June 2008.  In that case, a DNA test concluded that there was a 99.999 percent chance that the twins had different fathers. The twins' mother acknowledged having had intercourse with more than one man in a short period of time close to the conception date.
In another Greek myth, Leda conceives four children (Helen of Troy, Clytemnestra, and Castor and Pollux) in the same night by two different men. Two are children of Zeus, who comes to Leda disguised as a swan, and two are the children of Leda's mortal husband, Tyndareus. Which men father which children varies widely among accounts, in some cases establishing that the Gemini twins Castor and Pollux are born of different fathers. The heteropaternal superfecundation involved in this myth is especially unusual, because instead of giving birth to the children, Leda lays eggs that hatch them.
In Bryce Courtenay's "Australian" Trilogy the twins brothers Tommo and Hawk Solomon have different fathers, and were conceived on the same day.
On the NBC soap opera Days of our Lives, a character named Sami Brady has fraternal twins born of two different fathers after having sex with both men within hours of each other. Similarly, the character Blake Marler on CBS's Guiding Light initially believes her twins have different fathers, but they are later proven to both be her husband's biological children. On One Life to Live in 2001, longtime heroine Victoria Lord Davidson discovers her twin daughters have different fathers.
On his talk show, Maury Povich once discovered that a man was the father of one twin and not the other. The alleged father did a cartwheel after hearing each test result. Povich asked the mother, somewhat in disgust, "Do you know how close in time you must have had sex with two different men for this to happen?"