Although the pair were conceived from the same batch of embryos, they were born five years apart to parents Simon and Jody Blake.
Simon, 45, and his 38-year-old wife Jody had been trying to start a family without success when they 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 their son Reuben, weighing 9lbs 5oz on 9 December 2006.
[See also: Fitting a new arrival into the family]
The couple, from Cheltenham, Gloucestershire, decided to try for another child in March last year using the three remaining frozen embryos.
Two of the embryos did not survive the defrosting process, but the final embryo was successfully implanted.
Against the odds, Floren arrived on 16 November 2011 weighing 8lbs 12oz - two weeks before her due date - but five years after her twin brother.
So how were Mr and Mrs Blake able to have twins born five years apart?
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.
[Relevant: One mum's £50,000 IVF struggle]
Dr Akande explained: “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.
“In essence they haven't come from the same embryo but from the same batch of embryos. 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.”
Speaking about his two children, Mr Blake, a lecturer at University College Birmingham told PA: “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.”
Mother Jody, 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.
The couple said Reuben was a little apprehensive about having a sibling but has quickly settled into his role as the eldest.
Mr Blake said: “Since the day Floren was born, he's been really tender and loving with her. “
Reuben’s parents said he is aware of the special relationship he has with his seven-week-old sister but it would be a while before he fully understands.
His mother added: “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.
“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.”
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