Little Lillie and Lacie Ferguson are now healthy toddlers, aged two, and are meeting their milestones.
But when their mum Nicky was pregnant the situation was not so rosy.
Nicky, 42, was warned that the unborn babies had a potentially fatal heart condition diagnosed at 16 weeks - and stopping the heart of baby Lillie was the only way for Lacie to survive.
Just the day after Nicky learnt she was carrying baby girls, she was given the devastating news that Lillie was only getting 30per cent of the placenta.
Doctors warned that if laser surgery to separate them was not carried out, both babies could die due to twin-to-twin transfusion syndrome, a rare illness where one sibling deprives the other of the blood supply, reducing the nutrients received by the weaker twin.
A second diagnosis was also made, of intrauterine growth restriction, making one sister smaller than the other.
But Nicky and husband James, 41, were determined to give both girls a fighting chance.
The couple had to go for scans two or three times a week, at the Royal Infirmary of Edinburgh, due to the high-risk pregnancy.
And they told their three older children Liam, 22, Reese, 15, and Connor, 12, about the possibility that neither twin could survive.
But despite Nicky considering the possibility of stopping Lillie’s heart, she knew she would have had to give birth to her along with Lacie - and building site manager James urged her to give both girls a chance.
Nicky said: “From 16 weeks it was a bit of a nightmare - there was always something happening.
“Obviously I had said to my husband ‘what if we lose them both?’ - that was my biggest fear.
“But he was adamant they were fighters, and said ‘they came together, we need to see them out to the end’.
“I don’t think I could have physically gone through with the procedure.”
She had been advised to stop Lillie’s heart, to give Lacie a better chance of pulling through.
But at a 30 week scan, Nicky was suddenly told the risks were too great and a C-section needed to be carried out.
The consultant who had seen her all the way through the pregnancy even came back in to perform the op herself despite having clocked off her shift.
And three months after the babies were born, Nicky was invited to bring them into the consultant’s office, where she described them as ‘wee miracles’.
After being born on March 16 2017, the babies were kept in incubators in the intensive care unit.
Nicky was able to hold them the day they were born, and visited every day until they were allowed to return home - but found it heartwrenching to be told by medics ‘you’ve held them long enough now’.
Despite the uncertain prognosis during pregnancy, she was always honest with her other children about the twins’ chances of survival.
Nicky said: “I was always honest with my other children about what could happen.
“Every time I had a scan they were texting me, they were always really concerned.
“My older children absolutely dote on them, they are really good with them.”
For the twins’ second birthday, family members came over to celebrate with a party at home.
And Nicky said that the girls have never suffered any ill health despite their unusual circumstances in the womb.
She added: “They are healthy and happy children.
“They pick up viral things just like everybody else but they have never needed hospital treatment.
“They are walking now, Lillie was walking first. She’s 4lbs lighter than Lacie and always has weighed slightly less.
“If one is crying or hurts herself, the other is really concerned.
“They are talking now, they never stop.
“It is a huge relief after all the uncertainty.
“I was worrying about them meeting their milestones but they have done amazingly well.”