Reception class pupil Reuben Blake went back to school today, but his twin sister will have to wait another five years until she is old enough.
That is because, despite the fact the brother and sister were conceived from the same batch of embryos, they were born five years apart to parents Simon and Jody Blake.
Mr Blake, 45, and his 38-year-old wife had been trying to start a family without success and began fertility treatment in 2005.
During the medical process, five embryos were created and two implanted in Mrs Blake, which resulted in the birth of Reuben on 9 December 2006.
The remaining three embryos were frozen until the couple, from Cheltenham, Gloucestershire, decided to try for another child last year.
Against the odds, Floren arrived on 16 November 2011 - two weeks before her due date - but five years after her twin brother.
"I tell everybody I can," said Mr Blake.
"Just in kind of mundane settings where people take an interest in a newborn baby and with Reuben around as well, I find it very difficult to resist the temptation to say 'Oh and by the way they are twins'.
"It's almost just to see people's response. They are really amazed and surprised."
His wife, a programmes manager for the children's charity WellChild, said: "It does feel quite surreal. I think people are really, really surprised and it almost takes people a few minutes to get their heads around it.
"We obviously had nine months to get it straight and to think 'Gosh, we're having Reuben's twin', but it's incredibly special.
"When we had Floren, because I had her by Caesarean, we told the theatre team and they were all absolutely blown away and excited, ecstatic really.
"They were sort of saying 'We've never had this before' and they were just really, really excited for us, which just made the experience really special."
Reuben, who had been a little unsure about having a sibling and admitted he had wanted a brother, has quickly settled into his role as older brother.
"Since the day Floren was born, he's been really tender and loving with her," said Mr Blake, a business and economics lecturer at University College Birmingham.
Reuben, who is in the reception class at Christchurch Primary School in Cheltenham, said he had been looking forward to going back after the Christmas break.
Mrs Blake said: "It's nice that he is in a different phase. He's started school, which he loves. He's really happy there, he's got lots of friends there and equally it's nice for us to have lots of time with Floren when he is at school.
"He certainly likes to push her home from school in her pram and he also took her into school a few days after she was born and he was so excited to show all his friends, which is really, really nice."
Even at his young age Reuben is aware of the special relationship he has with his seven-week-old sister, although his parents said it would be a while before he fully understands.
"He knows that she's been in the freezer - he likes to say she has been in the freezer with the chips and the chicken - so he is sort of aware that she is his twin, but obviously he doesn't really understand how it's all worked really," his mother said.
"They do look very similar. Reuben was just a bigger version of Floren when he was born, so certainly there are similarities physically.
"She does look like a mini version of him really."
Mr Blake added: "Little bits of personality are similar as well. She's quite a feisty little character, she's quite vocal, so she's already given us signs she's going to be quite a strong personality and he's quite determined, independent, stubborn sometimes and assertive."
Mrs Blake, originally from Stroud, Gloucestershire, and her husband, from Brighton, East Sussex, met in 1999 when they both worked in the medical profession in London.
The couple, who have been married for nine years, had been trying to start a family for some time but without success.
As a result, they were referred to a fertility clinic in Bristol, which at the time was based in Clifton but has since moved to Southmead Hospital.
They were first referred to the Bristol Centre for Reproductive Medicine in September 2005 to have intracytoplasmic sperm injection (ICSI) treatment - a technique used with in-vitro fertilisation (IVF) in which one sperm is injected into one egg in order to fertilise it.
Five embryos were produced and two were implanted in Mrs Blake's womb. The remaining three were then frozen for a future date.
The treatment was successful and Reuben was born four days after his due date weighing 9lb 5oz.
In March last year the couple decided they wanted another child.
The three embryos were taken from the freezer where they had been kept but only one survived the defrosting process which was then implanted into Mrs Blake.
Thirty-nine weeks later Floren arrived by Caesarean section weighing 8lb 12oz.
Mr Blake said: "We got very, very lucky first time around. We were just overjoyed at having Reuben - he was healthy and happy.
"But after some time we thought that most of our friends who had conceived naturally had had second children and we wanted to complete the family.
"We were aware the odds were long. There aren't a huge amount of reliable statistics because it isn't done that often, but we knew it was a long shot.
"You just can't comprehend that a life could come from some material that's been frozen for that length of time."
Mrs Blake added: "I think the fact that we had three embryos in storage was also always at the back of our mind to use them at some point, but I don't think for one minute we thought we would be lucky with using those.
"It feels like an absolutely miracle really, and I think we were incredibly lucky.
"We remember the heartbreak of infertility and that never quite goes away but I think when you are so fortunate to have two very health children we do feel incredibly fortunate."
Her husband said the relationship between the two children was special for the whole family.
"It's something special for us and it's something special for them as siblings in the future when they are able to comprehend exactly what went on," he said.
"If we hadn't have gone through the process, we wouldn't have had Reuben, and he's just changed our lives, and then to have a sister - and a twin sister at that - it's always going to be extra special.
"We would definitely recommend it, but with a reality check all the way through, but I think most people that go through fertility treatment are juggling that all the time, it doesn't really leave you."
Doctors at the Bristol Centre for Reproductive Medicine, based at Southmead Hospital, said the decision made by Mr and Mrs Blake to freeze their remaining embryos was a safer way to have twins.
Dr Valentine Akande, lead clinician and director of fertility services, said: "We're delighted with the great outcome that Jody and Simon have encountered.
"It's a sensible approach to safely having babies.
"It's usually better to have one baby at a time rather than two because carrying twins is associated with greater risk.
"So we would very often recommend storing surplus embryos so that they can be used at a later date.
"Sadly, due to the chance work of nature, not everybody is able to have those surplus embryos and, of course, not everybody meets with success when those embryos are used."
Dr Akande explained the science behind the fact that Mr and Mrs Blake were able to have twins born five years apart.
"In essence they haven't come from the same embryo but from the same batch of embryos," he said.
"What's happened in this happy case is that a certain number of eggs were collected in one treatment cycle and all those eggs were fertilised to create embryos.
"Now, all those embryos at that stage could be considered twins, triplets, quadruplets or quintuplets, if they were all put back together at the same time and she had become pregnant, say with five.
"What's happened here, and the sensible thing to do, was to put back one or two earlier on and save those surplus ones for use at a later date, and that is exactly what has happened.
"So the surplus embryos were kept in storage and, when they were ready to use them, we brought them out of the freezer, saw that they were still healthy, and one of them survived and was put back into Jody's womb and we've got a baby.
"It does depend how you interpret the term 'twins' - twins generally means that they are born at the same time.
"But, yes, twins in that they have come from the same batch of embryos, collected from the same treatment cycle - so twins born at a different time - but not a twin pregnancy, when they have grown in the womb together."Suggest a correction