Superfetation is a rare phenomenon in which a woman becomes pregnant with a second fetus while already pregnant with another. This occurs when a woman ovulates and conceives a second time while already carrying a pregnancy. While extremely rare in humans, superfetation is more commonly observed in some animal species, such as rabbits and cats.

One notable case of superfetation occurred in 2009 when an Arkansas woman, Julia Grovenburg, became pregnant with two babies conceived two weeks apart. This rare occurrence garnered significant attention from the medical community and the public, highlighting the enigmatic nature of superfetation.

While the exact mechanisms behind superfetation in humans are not fully understood, it is believed to involve hormonal fluctuations that allow for the release of another egg and its subsequent fertilization, despite an existing pregnancy.

References:

• "Superfetation: A Case Report" - The Journal of Reproductive Medicine
• "Superfetation in Humans" - Case Reports in Obstetrics and Gynecology