Carols can evoke a range of emotions, from the festive blues to the heady anticipation of everything the Christmas season has to offer.

For Heather and Ian Harris, the sounds of the silent night transport them back to the intensive care centre for premature babies, where the thin beat of a Christmas number one from a hospital radio mingled with the beeps of medical equipment.

On this day in 1996, twins Sam and Holly were delivered just 25 weeks into Heather’s pregnancy and given narrow odds of survival. Today they celebrate their 16th birthday.

At 1am that day, Mrs Harris was dancing with colleagues at her work Christmas party not knowing that just hours later she would be in Watford General Hospital giving birth to two tiny babies.

Sam was born by emergency caesarian section at 9.18am, weighing 1lb 6oz, followed five minutes later by Holly, weighing 1lb 13oz.

They were then transferred to an intensive care unit for premature babies at Queen Charlotte’s Hospital in London.

Mrs Harris said: "The staff were absolutely honest with me and explained the twins’ chances of survival were not good because they were so small, and there was a 70/30 chance of brain damage.

"They were very, very tiny."

While the twins were in hospital, between January and March, Heather went back to work.

This required a 6am trip to Watford General Hospital to feed the babies, followed by a drive to her Heathrow office between 11am and 4pm, and then a drive back to the hospital until 11pm. Ian would travel from Milton Keynes to sit by the incubators overnight.

Now living in Ashley Green, near Berkhamsted, the twins are looking forward to a birthday party at an ‘all you can eat’ Chinese restaurant with 20 friends from school.

Mrs Harris said that only after the birthday celebrations are finished, is Christmas allowed to begin.

She added: "We’ve got a big family Christmas and the house is full of piles of Christmas presents and birthday presents. It’s the only time I think of the tough start they had.

"The rest of the time we’re rushing around like any other family. But when we were in hospital all we heard were carols. It just brings it all back to me. The overall feeling is how incredibly lucky we were.

"You wouldn’t think they’d had the start they did. They are never ill; they never even catch a cold."

Mrs Harris said despite being twins, Sam and Holly are completely different.

She added: "Holly is blonde, blue eyed and fair and Sam looks like Tom Daley. Holly is gregarious and loud, has loads of friends and always wants to be on the go. Sam much prefers sitting and reading or playing football. He’s the neat one and she’s the untidy one."

Sam, who has cerebral palsy, trains with Hertfordshire’s disability football team and is hoping to study sport at college next year.

Holly works at a stable at weekends and has enrolled in an animal studies course at another college.

Mrs Harris said: "We never thought at 16 they’d be heading to college. They have spent every moment together so it will be a challenge for them, and very strange for me, having always seen them as a pair."

Mrs Harris has run 11 marathons, and raised thousands of pounds, for the BLISS premature baby charity.

She added: "It’s been 100 times better than we thought it would. At the time people told us we were amazing, but you have no choice. You just have to get on with it.

"We both lost about three stone in weight, but we were their parents and they needed us to look after them.

"It was very negative at the beginning because the doctors didn’t want to give us false hope, but we feel we can give people real hope. Your children can surprise you."