Rosie and Ruby Formosa, both now four years old, were born joined at the abdomen and shared part of their intestines.
They beat the odds of survival after undergoing surgery as infants, and now they're celebrating a huge milestone -- their first day of kindergarten.
The girls, who were delivered by caesarean section at 34 weeks, posed in their new school uniforms in southeast London.
Their mother, Angela Formosa, told BBC News that the girls were very excited to start school. 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 [go] to [Great Ormond Street Hospital] really.
Conjoined twins is a condition that occurs in one out of every 200,000 live births.
The consultant pediatric surgeon, Professor Paolo De Coppi, at the hospital told BBC News,
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.
When talking about her daughters, Angela added,
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.