Julia Grovenburg and her husband Todd were shocked to learn she would be giving birth to two babies, but not twins.

The couple attended an ultrasound appointment believing Mrs Grovenburg was just over eight weeks pregnant.

But the scan showed two babies, separated by two-and-a-half weeks - with one clearly more developed than the other.

"[When] she said, 'and baby number two has got a healthy little heartbeat,' - I just started gagging," Mrs Grovenburg told broadcaster KFSM-TV in Arkansas.

"[We were] both in shock," her husband added.

He went on: "We were trying to put the timelines together and everything.

"We had known she had had a migraine and been at the hospital and actually had a pregnancy test at the time that one would've been conceived..."

His wife continued: "We feel blessed to have something so rare and as of this point they're perfectly healthy."

Doctors suspect an extraordinarily uncommon situation called superfetation, which means conceiving when already pregnant.

Patrick O'Brien, consulting obstetrician and spokesman for the Royal College of Obstetricians and Gynaecologists, told Sky News Online only one case a year is reported worldwide.

"It's a rare thing because when you conceive, your hormones change dramatically," he said.

"Those changes stop you ovulating and they stop you conceiving."

Tests will be performed when the babies are born to ascertain what happened.

But Dr O'Brien added that the Grovenburgs may not ever know for sure, because in their case the tests could not rule out twins.

"It's hard to be certain sometimes because a woman could be having non-identical twins that are markedly different in size from early in a pregnancy," he said.

"If there's a big size difference between the twins, the first suspicion is that one is not developing as well."

Superfetation became a more accurate suspicion when the babies appeared to be more than two weeks apart, he added.

If the family's situation was not astonishing enough, the babies are officially due in different years - at the end of 2009 and start of 2010.

However, the pair - already named as Jillian and Hudson - were expected to be born together, either naturally or by caesarean section, in December.