In fact, thought they share the same birthday, the little girls were conceived an entire three weeks apart thanks to a million-to-one medical rarity called superfetation, where the body carries on releasing more eggs for fertilisation after a pregnancy has already occurred.

Mum Amelia Spence admits that she and partner George Herrity were as stunned as the doctors when they first found out that she was carrying two children, but they were not twins.

"It is hard to get our heads around the fact that I was pregnant with two babies at the same time and they aren't twins," the Daily Mail quoted her as saying.

"We looked at the screen and the doctor pointed out that one foetus still looked like a blob, whereas the other had four visible limbs. It was such a marked difference.

"It was really strange to see. The doctor said that he had never seen it before, but had only read about it in textbooks.

"One foetus was 12 weeks old, whilst they said the other was three weeks behind, at nine weeks old. I had become pregnant twice on two separate occasions.

"The doctors couldn't believe it, either. They told us there was a million-to-one chance of it happening.