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.
This situation can also be seen when twins are the result of fertility treatments, for example the case of Koen and Teun Stuart, Dutch boys who were the result of in vitro fertilization (IVF). In a mixup at the laboratory, equipment had been used twice, causing another man’s sperm to be mixed with the intended father's.
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.