Harrison Papukchi was born weighing just 2lb 2oz on September 25 - 15 weeks earlier than expected.
He had an identical twin brother Ellison, who weighed only half of what Harrison did, but he died at five weeks old after complications following the emergency C-section birth.
Mum Abby Tighe had been diagnosed with twin-to-twin transfusion syndrome (TTTS) - a life-threatening condition that affects around ten per cent of identical twins who share a placenta.
And Ellison, who had two operations, suffered from bowel problems that meant he did not survive.
Abby, 24, and partner Ezel, 25, were finally allowed to take Harrison home on New Year’s Eve - three months after his traumatic start to life.
And Abby said Harrison has since been ‘fantastic’ - he came off oxygen for the first time four months ago and has no more cardiology or respiratory appointments planned.
He still goes to physiotherapy twice every three months to strengthen his core muscles which are weaker than typically-developing babies.
“He’s learning, laughing and giggling every day, he’s a very happy baby,” said Abby, of Station Road, Wombwell.
“I can’t believe he’s one year old, we don’t know where the time’s gone.
“Harrison surprises me every day.
“It’s mixed emotions, because I’m obviously happy and grateful he’s doing well but also still thinking about his twin brother.
“We’ve had good days and bad days, sometimes you don’t know whether to smile or cry.
“But we’re focused on the now.”
The family had a huge birthday bash for Harrison at Hillies Golf Club, Wombwell, on Sunday before his birthday on Wednesday.
Abby said: “It was fantastic, everybody turned up who we wanted to and Harrison enjoyed it.
“Everybody wanted to hold him, he was passed around like a little parcel but he didn’t mind.
“We put a picture of his brother next to his birthday cake so again it was very mixed emotions, but we just want to look 100 per cent forward now.”
The family buried Ellison with Abby’s granddad, and created a memory box that they could show Harrison when he’s old enough to understand.
They visited the grave on Harrison’s birthday, and while his twin brother will always be remembered, Abby said the couple are focused on bringing up their ‘precious’ baby boy.
Abby said the family might even get to go on their first holiday together, to Bulgaria, where Ezel is originally from, next year.