ONE was twice the size of the other and three weeks older - but they were both born on the same day.

It’s hard to believe these babies are twins but that is because they are medical wonders.

In an incredibly rare phenomenon known as superfetation, the twins from Trowbridge were conceived three weeks apart.

Somehow, their mum, Rebecca Roberts, 39, became pregnant with Rosalie while already pregnant with Noah. Only 14 such cases have been documented worldwide - and the twins are one of them.

Miss Roberts, from Hilperton, was stunned when she found out she was having twins, as was her partner Rhys Weaver, 43, when she told him.

Rebecca said: “We found out we were pregnant in February, so no lockdown baby jokes there.

“I was offered an early scan at seven weeks, Rhys was able to come to the scan too as the hospitals were still open to partners at that point.

“We saw our tiny little baby, just! It was so small and we were told we were expecting one baby.

“I was also given another scan at ten weeks to check everything was A OK and there was our little baby wriggling around.

“Fast forward a few weeks and I had my 12-week scan. Unfortunately, Rhys wasn’t allowed in the hospital as lockdown had begun.

“The stenographer said 'Do you know you are having twins?' The scan said Twin 1 and Twin 2. I was so happy.

"That was amazing news but things got even more amazing.

“It was very obvious that one twin was much bigger than the other. There was a three-week size difference between them both.

“They could not understand what was going on and why the size difference was so big.

"After multiple scans the twin specialist at Bath RUH came to the conclusion that I had ovulated again. So basically I got pregnant whilst being pregnant. It’s a totally rare phenomenon called Superfetation.”

Miss Roberts was sent to St Michael’s Hospital in Bristol where one of the specialists said she thought it was not Superfetation and that something had happened to Rosalie in early development.

The twins were delivered early by caesarean at nearly 34 weeks on September 17 because of a problem with Rosalie’s umbilical cord. While Noah weighed 4lb 10oz, tiny Rosalie was half his size at just 2lb 7oz.

Afterwards, Rosalie was taken to St Michael’s for a medical problem, which cleared itself, and tests for various syndromes which all came back clear.

She came back to Bath and was kept in the RUH neonatal intensive care unit for up to 95 days, finally being discharged home at Christmas.

Miss Roberts, who also has elder daughter Summer, 14, has also had tests to rule out chromosomal or genetic reasons for the rare phenomenon.

She and Mr Weaver have been told a fertility drug she was taking may have caused her to release an egg while already pregnant.

She said: “The care from the NICU staff towards Noah, Rosalie and ourselves was outstanding, I can’t fault it.”

“Noah is really thriving and so is Rosalie, she’s growing well.”

Because of the rare phenomenon, the couple have set up an Instagram account for the twins where people can follow their progress.

They now have a fan base of more than 16,000 followers worldwide. To follow them, go to @roberts.supertwins