As they happily show off their uniforms before their first day at school, it is hard to believe identical twins Rosie and Ruby were once given almost no chance of survival.
Before the girls were born, their parents Angela and Daniel Formosa were told they were joined at the abdomen.
There was a high risk that they would not survive the pregnancy, and if they did then they might not survive the birth or the subsequent surgery to separate them.
"I didn't prepare to bring them home," said Angela Formosa, of Bexleyheath, South-East London.
Thanks to the skills of a team of doctors at Great Ormond Street Hospital, however, the girls were successfully separated when they were only a few hours old.
They have thrived ever since and now, aged 4, they are 'very excited' about starting school - a milestone which seemed impossible back in 2012.
Angela Formosa, 35, 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 Great Ormond Street Hospital."
Conjoined twins occur every 200,000 births. As few as 5 per cent of those births actually survive being born.
The parents were warned the girls had a very low chance of survival when doctors made the discovery when Formosa was 16 weeks pregnant.
"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 mother said.
The girls, who shared part of the intestine, were born at University College Hospital in London by caesarean section when Formosa was 34 weeks pregnant.
Within a couple of hours of being born, they were taken to world-renowned Gosh for emergency surgery which lasted five hours and involved 15 doctors because of a life-threatening intestinal blockage.
The girls, who were born weighing 5lb 3oz each, amazed doctors by doing so well that they were healthy enough to go home at just three weeks old.
Taxi-driver Daniel Formosa, 40, said he had feared the girls would not make it with such slim odds against them.
"It was very difficult when we first found out, it was touch and go so at that stage every day was a battle.
"We did not ever think we would come this far, but they are amazing and this is another milestone they have achieved," he added.
Angela Formosa 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. 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 celebrates children who are able to go to school thanks to Gosh 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.