One of the fun benefits of trying for a baby is that you no longer have to worry about that pesky contraception. Naturally, one would assume it's pretty much unnecessary during pregnancy too, but this story has us second guessing that assertion.
As it turns out, it is possible to get pregnant while you're already pregnant.
And yes, you are reading that correctly. It's all down to a (thankfully rare) phenomenon called superfetation (also spelled superfoetation), which is the 'simultaneous occurrence of more than one stage of developing offspring'.
In mammals, it manifests when a second egg is released and fertilised as an embryo from a different cycle is already developing in the uterus.
While there are risks (the second baby is often born prematurely, which can increase its odds of experiencing development problems) there are cases of women giving birth to two healthy babies conceived weeks apart.
The first recorded case is in 1960, when Mary Tress of Baltimore in the USA had what the nurse mistakenly identified as twins. When doctors delivered the boys, Anthony John and Mark Francis, they noted that Anthony, born five minutes before his brother, appeared to be premature.
When X-rays of the boys' thigh bones were taken, they revealed that the boys had different bone ages. Anthony, a two-month premature baby, had been born five minutes before his full-term brother Mark, but was conceived a full two months later than his brother.
In 2007 UK couple Amelia Spence and George Herrity welcomed Ame and Lia Herrity, conceived three weeks apart, while Harriet and Thomas Mullineux, also conceived three weeks apart, were born in Essex to Charlotte and Matt Mullineux.
Arkansas couple Todd and Julia Grovenburg found themselves in the headlines in 2009 when their babies, conceived two and a half weeks apart, were delivered by C-section.
Last year, doctors confirmed that a pair of twins born in Vietnam have different fathers, effectively making them half-siblings. 
