Imogen and Annabelle Weir were born at just 23 weeks and four days in Glasgow meaning their mother could still have legally had an abortion.
Under law, a mother is allowed to have a termination at 24 weeks because scientists believe that under this point life outside the womb is not viable.
The twin's mum Claire told the BBC: "The consultant told us that if the girls had been born just two years ago, they wouldn't have survived - that's how fast medical technology is advancing.
"They have surprised everyone. They will always be our little miracles."
Peter Williams from Right to Life told Premier: "I think it shows that as medical science continues to improve and improve, the so-called time of viability where a baby can be born alive and survive outside the womb, is going to be pushed lower and lower."
He added: "What this should really tell us is that the abortion limit as we have it, given the humanity of the unborn child, this arbitrary limit of 24 weeks should really be pushed back further".
He said whilst he wanted abortion outlawed completely, a good start would be reducing the limit to 20 weeks.
According to government figures, the number of woman who have abortions after being 13 weeks pregnant is just eight per cent.
The statistics show that 92 per cent of terminations happen before 13 weeks.