Here's how it's possible to get pregnant when you're already pregnant

File this story under 'WTF news'

If you thought 'double pregnant' was a turn of phrase employed by meme artists and overly-amorous men, then think again.

It turns out that it is possible to get pregnant when you're already pregnant.

But it's pretty rare. According to a piece published this week in The New York Times, there are fewer than 10 cases of double pregnancy - or superfetation - ever recorded.

One of these cases is an American woman called Julia Grovenburg, who discovered she was pregnant back in 2009 and went in for a routine scan. The scan showed that her unborn daughter had another, smaller baby growing next to her. Incredibly, the other baby had been conceived a few weeks after her daughter.

Both babies - Jillian and Hudson - were born by C-section on the same day, but medically speaking the two are not twins as Hudson is actually two weeks younger than Jillian.

How does superfetation happen? Normally when a woman gets pregnant there are hormones which automatically stop another pregnancy from happening, the biological equivalent of 'sorry we're fully booked right now.'

For another baby to be conceived by a pregnant woman this hormone signal has to fail. Then after that it's just a simple matter of semen making its way through a cervix that has been sealed to fertilise an egg that's been released after the woman has fallen pregnant and for this fertilised egg to make itself comfortable in a uterus lining that's already technically 'taken.'

After reading the above you can probably see why cases of superfetation are so rare. Though apparently it's pretty common in mammals such as cats.

The irony of the Grovenburg situation? The couple had been trying unsuccessfully to have a baby for three years and had nearly given up when she fell pregnant.

How does that Wendy Cope poem go again?