In mammals, it manifests as the formation of an embryo from a different menstrual cycle while another embryo or fetus is already present in the uterus. When two separate instances of fertilisation occur during the same menstrual cycle, it is known as superfecundation.
While proposed cases of superfetation have been reported in humans, the existence of this phenomenon in humans has been deemed unlikely. Better explanations include differential growth between twins due to various reasons such as twin-to-twin transfusion syndrome. Artificially induced superfetation has been demonstrated, although only up to a short period after insemination.
A 2008 French study found evidence to suggest that superfetation is a reality for humans, but that it is so rare that there have been fewer than 10 recorded cases in the world.
In 2017, it was reported that an American woman who had agreed to act as a surrogate for a Chinese couple bore two babies initially believed to be twins. Before the adoptive parents could return home to China, however, it was discovered that one of the babies was, in fact, the biological son of the surrogate. Doctors confirmed that the birth-mother had become pregnant with her and her partner's child roughly three weeks after becoming pregnant with the Chinese couple's child.
There have been multiple cases reported to local US doctors with a week or less difference in age of twins and women who report two surges of ovulation occurring within a few days of each other. Though rare, this condition is believed to affect as many as 0.3% of women but often one twin is lost so the true numbers are not known. Research has found 10% of women released two eggs in a cycle, but both at the end of the same "wave" of follicullogenesis, which doesn't actually support the theory of superfetation in humans.
In September 2020, a woman in Wiltshire, England, gave birth to fraternal twins who were conceived three weeks apart.
Animals that have been claimed to be subject to superfetation include rodents (mice and rats), rabbits, horses, sheep, marsupials (kangaroos and sugar gliders), felines, and primates (humans). Superfetation has also been clearly demonstrated and is normal for some species of poeciliid fish.
- Roellig, K; Menzies, BR; Hildebrandt, TB; Goeritz, F (February 2011). "The concept of superfetation: a critical review on a 'myth' in mammalian reproduction". Biological Reviews of the Cambridge Philosophical Society. 86 (1): 77–95. doi:10.1111/j.1469-185X.2010.00135.x. PMID 20394608.
- McNamara, HC; Kane, SC; Craig, JM; Short, RV; Umstad, MP (February 2016). "A review of the mechanisms and evidence for typical and atypical twinning". American Journal of Obstetrics and Gynecology. 214 (2): 172–191. doi:10.1016/j.ajog.2015.10.930. PMID 26548710.
- Fletcher, Dan (28 September 2009). "How Can a Pregnant Woman Get Pregnant Again?". Time Magazine. Retrieved 29 March 2018.
- O. Pape, N. Winer, A. Paumier, H.J. Philippe, B. Flatrès, G. Boog (4 June 2008). "Superfœtation: about a case and review of the literature". EM Consulte Journal of Obstetrics and Reproductive Biology.CS1 maint: multiple names: authors list (link)
- "Surrogate Mom Gets Pregnant While Carrying Another Couple's Baby". Insideedition.com. 30 October 2017. Retrieved 16 December 2018.
- Vince, Gaia. "Women can ovulate more than once a month". New Scientist. Retrieved 22 October 2018.
- "Woman gets pregnant while already pregnant, gives birth to twins conceived 3 weeks apart".
- "FishBase Glossary". Fishbase.org. Retrieved 16 December 2018.