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.
The four-year-olds, from Bexleyheath in Kent, are “very excited” to be starting school like their big sister Lily, nine, Ms 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 didn’t prepare to bring them home. It wasn’t until they were in hospital and had their operation that my husband started painting the bedroom and getting everything ready for them.”