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Superfecundation is the fertilization of two or more ova from the same cycle by sperm from separate acts of sexual intercourse. The term superfecundation is derived from fecund, meaning the ability to produce offspring.
Heteropaternal superfecundation occurs when two different males father fraternal twins. When the parents of fraternal twins are involved in a paternity suit, the odds are about 1 in 40 that the children are actually half-siblings.
In common usage, the term superfecundation is often used instead of heteropaternal superfecundation. The terms are practically equivalent because, though it is speculated that superfecundation by the same father is a common occurrence, it can only be proven to have occurred when there are multiple fathers.
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 female'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 common in animals such as cats and dogs. Stray dogs often produce litters in which every puppy has a different father. Though rare in humans, cases have been documented. In one study on humans, the frequency was 2.4% among dizygotic twins whose parents had been involved in paternity suits.
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.