Can a woman get pregnant… when she is already pregnant?

A few days ago, a question and answer session orchestrated by the New York Times took an ominous turn that goes against everything we learned in sex ed class by confirming that it is indeed possible for a woman to become pregnant… when she is already pregnant!

Superfetation is the implantation of a new pregnancy into a uterus that already contains a developing pregnancy. In other words, in very exceptional circumstances, a woman can continue to ovulate during pregnancy and conceive another child. It is extremely rare in humans, a case was publicized in Arkansas in 2009 and a second in Australia in 2015. In 2009, a woman had actually become pregnant after having already conceived two and a half weeks earlier. The two babies were born by caesarean section and in good health on the following December 2, even if the second was a fortnight premature. In 2015, an Australian woman also gave birth to two daughters, one of whom was ten days premature.

But then, how is this possible? Superfetation is actually quite common in mammals other than humans. It is particularly observed in rodents, rabbits, horses, sheep or even in marsupials such as kangaroos. Sometimes these animals have two wombs to facilitate double pregnancy or sometimes their menstrual cycle simply continues during pregnancy. But in humans, superfetation appears to be a very rare « accident » — a pregnant woman’s body has normally evolved to prevent a second pregnancy from occurring.

 » Normally, egg release stops once the woman is pregnant. “, explains C. Clairborne Ray to the New York Times, “ pregnancy-induced hormonal and physical changes prevent further conception. But for some reason, in superfluity, a pregnant woman still manages to ovulate « . The sperm must then be able to “find its way” and fertilization follows, which is an extremely delicate process even in ordinary pregnancies. Only when a woman is already pregnant, her hormones would normally make the uterus an unfavorable and inhospitable environment for another fertilized egg.

 » For superfetation to occur in humans, it would seem that three seemingly impossible things must happen. », Explains this time Khalil A. Cassimally for Scientific American.  » Ovulation must take place during an ongoing pregnancy, the sperm must somehow “find its way” through the blocked cervix to the oviduct and finally successfully implant into an already occupied uterus « .

Given the low number of reported and verified cases, it is currently not known why superfetation sometimes occurs and whether there are risk factors that can increase the chances of the phenomenon.


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