DOCTORS believe that "twins" born to a North-East mother have defied odds of a million to one.

Beverley Robson, 32, from Bishop Auckland, County Durham, gave birth to dark-haired Leah and blonde-haired Lara on the same day.

But scans suggest that unlike twins, the two babies were conceived separately and developed as two different embryos.

In effect, Mrs Robson got pregnant twice over.

Even more remarkably, both babies survived and are doing well.

Specialists at Darlington Memorial Hospital believe that Leah and Lara are examples of a phenomenon known as superfecundation.

This term is used when two eggs from the same cycle are fertilised separately.

Experts believe that the two babies were conceived two or three weeks apart.

Normally, a second fertilised egg fails to implant in the womb and leads to a miscarriage.

Even on its own, this phenomenon is extremely rare, but the fact that both babies have survived has astonished doctors.

"They told me at the beginning it was really unusual, that it was one in a million," said Mrs Robson.

"They said it was very rare that the second baby survives," she added.

At first, doctors were puzzled because the two babies were different sizes.

"From my six weeks scan, they noticed a big difference between them," she said.

When doctors suspected superfecundation they warned Mrs Robson and her husband, Craig, 29, that it was rare for a second baby conceived in this way to survive.

"I couldn't do anything about it. I just had to rest all the time, take it easy and hope for the best," she said.

Bob Aitken, medical director for the County Durham and Darlington Foundation NHS Trust, said: "From the scans we did we think that Beverley's babies could have been conceived at different times, although we can't be absolutely certain.

"I've only come across one case in 30 years of delivering babies, so it is very rare."

Mrs Robson gave birth to the two girls by Caesarean section on September 26.

Leah, the older girl, weighed in at 6lb 6oz, with her younger sister weighing 4lb 4oz.

Mrs Robson said: "We are thrilled to bits. I have already got two boys and I have always wanted a girl. Now our family is complete."