Rosie and Ruby Formosa, who were born joined at the abdomen and shared part of the intestine, needed an emergency operation to separate them when they were born in 2012.
Their parents, Angela and Daniel Formosa, were told the girls had a low chance of survival when medics discovered they were conjoined.
But after a successful separation operation at London’s Great Ormond Street Hospital (Gosh), the identical twins lead happy and healthy lives and are preparing to start school next week.
The four-year-olds, from Bexleyheath in Kent, are “very excited” to be starting school like their big sister Lily, nine, Mrs Formosa said.
“Four years ago it wasn’t in my mind that this would ever happen,” she said.
“When I was pregnant I didn’t think I’d ever see their first day at school so it is really amazing and all thanks to Gosh really.”
Mrs Formosa said it was “heartbreaking” when she discovered the girls had the rare medical condition - it accounts for one in every 200,000 live births.
“At 16 weeks they sent me to King’s College Hospital and it was there that they discovered the connection between the girls,” she said.
“It was heartbreaking really - I was already worried that they were monoamniotic (where twins share an amniotic sac), and conjoined was the worst-case scenario.
“I was really, really, really scared and really upset because at that point I was told that there was a high possibility that the girls wouldn’t survive the pregnancy.
“And if they did survive the pregnancy they might not survive the birth, then they might not survive surgery.
“They couldn’t tell what was connecting them.
“I didn’t prepare to bring them home. It wasn’t until they were in hospital and they’d had their operation that my husband started painting the bedroom and getting everything ready for them.”
The girls were born at University College Hospital in London by caesarean section when Mrs Formosa was 34 weeks pregnant.
Within a couple of hours of being born, they were taken to Gosh for emergency surgery because of an intestinal blockage.
Praising the staff at the world-renowned children’s hospital, Mrs Formosa added: “They had a look and did scans and all sorts of tests and it wasn’t until they got into surgery that they saw what was going on.
“It was on-the-spot decisions as to what was to be done.”
The operation to separate them took five hours and the girls were well enough to go home when they were just three weeks old.
Mrs Formosa, 35, said it feels like “a million years” ago since she was waiting for the girls to come out of their surgery.
“The time has just flown by, I can’t believe how fast it has gone,” she said.
“They are very excited (about starting school); their big sister is in school so they can’t wait.
“They’ve met their teacher a few times and they love their teacher.
“They’re looking forward to painting, anything messy, they love reading.
“They are very similar, they are very bubbly little girls, they are very headstrong and very determined, which I knew they were from when they were in my belly because of the way they kept growing and surviving.
“I knew they were going to be determined and they are. They rule the roost.”
Great Ormond Street Hospital is the leading centre in Europe for the care of conjoined twins, performing the first successful separation surgery on conjoined twins in 1985.
It has since cared for 27 sets of conjoined twins.
The Formosa family are supporting the hospital’s charity through it’s Back To School Campaign - which is celebrating all of the children who are able to go to school thanks to care at the hospital as well as raising funds.
The campaign encourages people to share their children’s back-to-school moments on their social media pages to help raise money for Great Ormond Street Hospital Children’s Charity.
Professor Paolo De Coppi, consultant paediatric surgeon at Gosh, said: “Over the last 30 years we have treated 27 sets of conjoined twins at Great Ormond Street Hospital.
“The surgery is highly complex and requires teams from across the hospital to work together and combine a whole range of expertise.
“We’re thrilled that Rosie and Ruby are starting school this September.
“It’s always a joy to witness patients’ progress and to hear that they are reaching new milestones - this makes the job we do all the more rewarding.”
Tim Johnson, chief executive of Great Ormond Street Hospital Children’s Charity, said: “Thanks to the world-class care given to seriously ill children from across the UK at Gosh, more children will go back to school or enjoy their first day at school.
“We’re encouraging people from across the UK to share their back to school or first day at school moments and donate to help raise money for the hospital. Text SCHOOL to 70020 to give £3.”