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.
Superfetation is claimed to be common in some species of animals. In mammals, it can occur only where there are two uteri, or where the estrous cycle continues
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 fishes.
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.
In 2017, it was reported that an American woman who had agreed to act as a surrogate for a Chinese couple birthed 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 but both at the end of the same "wave" of follicullogenesis, which doesn't actually support the theory of superfetation in humans .
- 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.
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