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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, which can lead to twin babies from two separate biological fathers. The term superfecundation is derived from fecund, meaning the ability to produce offspring. Heteropaternal superfecundation refers to the fertilization of two separate ova by two different fathers. Homopaternal superfecundation refers to the fertilization of two separate ova from the same father, leading to fraternal twins. While heteropaternal superfecundation is referred to as a form of atypical twinning, genetically, the twins are half siblings. Superfecundation, while rare, can occur through either separate occurrences of sexual intercourse or through artificial insemination.
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 three 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 in mammals
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.
Cases in mythology
Greek mythology holds many cases of superfecundation:
- Leda lies with both her husband Tyndareus and Zeus, the latter in the guise of a swan. Nine months later, she bears two daughters: Clytemnestra by Tyndareus and Helen by Zeus. This happens again, this time Leda bears two sons: Castor by Tyndareus and Pollux by Zeus.
- Alcmene is raped by Zeus, who is disguised as her husband Amphitryon; the real Amphitryon later sleeps with Alcmene and she gives birth to two sons: Iphicles by Amphitryon and Heracles by Zeus.
Selected cases involving superfecundation
In 1995, a young woman gave birth to diamniotic monochorionic twins, who were originally assumed to be monozygotic twins until a paternity suit led to a DNA test. This led to the discovery that the twins had different fathers.
In 2001, a case of spontaneous monopaternal superfecundation was reported after a woman undergoing IVF treatments gave birth to quintuplets after only two embryos were implanted. Genetic testing supported that the twinning was not a result of the embryos splitting, and that all five boys shared the same father.
In 2016, an IVF-implanted surrogate mother gave birth to two children: one genetically unrelated child from an implanted embryo, and a biological child from her own egg and her husband's sperm.
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