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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.
In common usage, the term superfecundation is often used instead of heteropaternal superfecundation. The terms are practically equivalent because, though superfecundation by the same father is thought to be a common occurrence, it can only be proven to have occurred with multiple fathers.
Superfecundation most commonly happens within hours or days of the first instance of fertilization with ova released during the same cycle. The time window when eggs are able to be fertilized is small. Sperm cells can live inside a female's body for four to five days. Once ovulation occurs, the egg remains viable for 12–48 hours before it begins to disintegrate. Thus, the fertile period can span five to seven 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, a chance of a second pregnancy occurs, 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 can produce litters in which every puppy has a different sire. 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.
Use in mythology
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.
- James, William H. (1980). "Gestational age in twins". Archives of Disease in Childhood 55 (4): 281–284. doi:10.1136/adc.55.4.281. PMC 1626817. PMID 7191240. Retrieved 2011-03-17.
- Wenk RE, Houtz T, Brooks M, Chiafari FA (1992). "How frequent is heteropaternal superfecundation?". Acta geneticae medicae et gemellologiae 41 (1): 43–7. PMID 1488855. "The frequency of HS among dizygotic twins whose parents were involved in paternity suits is 2.4%"
- "Is it possible for fraternal twins to have different fathers?" at The Straight Dope
- Lu et al. (July 1994). "Paternity Identification in Twins with Different Fathers". Journal of Forensic Science 39 (4): 1100–1102.
- Gawker (18 May 2009)
- Doctors Discover Dallas Twins Have Two Different Dads