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Can a woman get pregnant while she is already pregnant?

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A few weeks ago, a questions and answers session orchestrated by the New York Times took a turn for the unusual, going against everything we learned in sex education classes, and confirmed that it is in fact possible for a woman to get pregnant while she is already pregnant! 

‘Superfetation’ is the implantation of a new pregnancy in a uterus that is already in the course of a developing pregnancy. In other words, in highly exceptional circumstances, a woman can continue to ovulate while she is pregnant, and can conceive another child. It is extremely rare in humans, but a case of this happening in Arkansas made the media in 2009, and another in Australia in 2015. In 2009, a woman got pregnant after having already conceived a child two and half weeks beforehand. The both babies were born by Caesarean section and in good health on the following 2nd December, although the second baby was around 15 days premature. In 2015, an Australian woman also gave birth to two daughters, one of them around a dozen days premature.

So how can this be possible? Superfetation is in fact fairly common in mammals other than humans. It is often seen in rats, rabbits, horses, sheep and even marsupials such as kangaroos. Sometimes, these animals have two uteruses to facilitate the double pregnancies, and sometimes their menstrual cycle simply continues during pregnancy. But in humans, superfetation appears to be a highly rare “accident” -a woman’s body is normally evolved to prevent another pregnancy from happening when she is already pregnant.

“Ordinarily, the release of eggs ceases once a woman is pregnant”, explains C. Clairborne Ray to the New York Times. “The hormonal and physical changes of pregnancy work together to prevent another conception.” The sperm usually has to “find its way” and fertilisation is an extremely delicate process even in normal pregnancies. And when a woman is already pregnant, her hormones should normally make the uterus an inhospitable environment for another fertilised egg.

“In order for superfetation to occur in humans … it would appear that three seemingly impossible things need to happen,” explains Khalil A. Cassimally to Scientific American“Ovulation must take place during an ongoing pregnancy, semen must somehow find its way through the blocked cervix to the oviduct, via the occupied uterus and finally, the conceptus has to successfully implant itself in an unsuspecting already-occupied uterus.”

Given the small number of cases reported and verified, we don’t yet know why superfetation sometimes occurs, and if there are risk factors that could increase the risk of this happening.

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