Superfetation in Humans - Myth or Reality?
Superfetation is the simultaneous occurrence of more than one stage of developing offsprings in the same animal. In mammals, it manifests as the formation of an embryo from a different estrous cycle while another embryo or fetus is already present in the uterus. Superfetation is claimed to be common in some species of animals, but is extremely rare in humans. In mammals, it can occur only where there are two uteri, or where the estrous cycle continues through pregnancy.[1,2,3]
Superfetation in humans is an extremely rare situation in which a woman becomes pregnant a second time with another (younger) fetus in the face of an ongoing pregnancy.  It is characterized by the fertilization and the implantation of a second oocyte in a uterus already containing the product of a previous conception. Superfetation is different from the process of twinning or multiple gestation and involves the conception of an additional fetus during an established pregnancy. With superfetation, the two fetuses have different gestational ages and due dates.  It can occur in some animals but is so rare in humans that fewer than 10 cases have been reported in the medical literature. Superfetation should not be confused with situations in which two separate instances of fertilisation occur during the same menstrual cycle, and this latter scenario is known as superfecundation. The risk with superfetation in humans is that the second baby is often born prematurely, which can increase its odds of experiencing lung development problems.
Animals that have been claimed to be subject to superfetation include rodents (mice and rats), rabbits, horse, 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.
In humans, reports of superfetation occurring long after the first impregnation have often been treated with suspicion, and some have been clearly discredited. Other explanations have been given (and demonstrated) for different levels of development between twins. Artificially induced superfetation has, however, been demonstrated, although only up to a short period after insemination.
The argument therefore remains whether superfetation in humans is a myth or reality? What makes the situation more challenging is that cases of superfetation "can only be confirmed after delivery by chromosomal and metabolic studies on the baby."  The query therefore is how Well-Documented are Cases of Superfetation?
While not every story can be considered reliable, superfetation seems to have occurred in a number of well-documented cases, and the case scenarios below are therefore insightful.
Roellig KL, Menzies BR, Hildebrandt TB, Goeritz F. The concept of superfetation: a critical review on a 'myth' in mammalian reproduction. Biol Rev Camb Philos Soc. 2011 Feb;86(1):77-95.
Ibrahim UN, Dauda M, Khan N, Okon IE. Superfetation in a double uterus - A case report. Niger Med J 2009;50:52-3.
Singhai S. R., Agarwal U., Sharma D., Sen J. Superfetation in uterus pseudo didelphys: an unreported event Arch Gynecol Obstet 2003; 268(3): 243-4.
Tuppen, G.D., Fairs, C., de Chazal, R.C. & Konje, J.C. Spontaneous superfetation diagnosed in the first trimester with successful outcome. Ultrasound in Obstetrics and Gynecology 14, 219-221 (1999).
Jacob Stephanie (September 2009). “Superfetati on Double Pregnancy”. AOL Health. Retrieved 1 September 2009.
Baijal N, Sahni M, Verma N, KumarA, Parkhe N, Puliyel JM. Discordant twins with the smaller baby appropriate for gestational age-unusual manifestation of superfetation: a case report. BMC Paeditr 2007-,197: 2.
Newsweek Magazine, 31 July. 1961. This rare case is called *superfetation* http://-en.wiki pedia.org/wiki/Superfetation.
“Mother Deliveries Babies Minutes Apart but They are Not Twins!", Medindia, 1 October 2007. This rare case is called *superfetation* http://en.wikipedia.org/wiki-/-Superfeta tion.
Hale, Beth. "The babies born just a minute apart who AREN'T twins" , The Daily Mail, Associated Newspapers, Ltd, 15 January 2008. This rare case is called *superfetation* http://en.wikipedia.org/wiki/Superfetation
Pape O et al. Pregnant and Pregnant Again: Superfetation J Gynecol Obstet Biol Reprod (Paris) 2008; 37(8): 791-95.
Rabinerson D et al. Pregnant and Pregnant Again: Superfetation Harefuah 2008; 147(2): 155-58.
Van Sipma, Ashley (1 July 2011). "I got pregnant while I was already pregnant! Woman gives birth to two babies on the same day but they are NOT twins". MailOnline. Archived from the original on 1 November 2011. Retrieved 1 November 2011.
Barns, Sarah "A WOMAN has conceived two baby girls 10 days apart." The Sun. This rare case is called *superfetation* http:// en.wikip edia.org/wiki/Superfetation
Roellig, K., Goeritz, F., Fickel, J., Hermes, R., Hofer, H. & Hilderbrandt, T.B. Superfetation in mammalian pregnancy can be detected and increases reproductive output per breeding season. Nature Communications. 2010; 1:78.
Cassimally, K. "Superfetation: Pregnant while already pregnant." Scientific American. Accessed January 7, 2016. Http://blogs. Scientificamerican.com/guest-blog/superfetationpregnant-while-already-pregnant.
Harrison, A., Valenzuela, A., Gardner, J., Sargent, M. & Chessex, P. Superfetation as a cause of growth discordance in a multiple pregnancy. The Journal of Pediatrics. 2005 147:2, 254-255.
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