After it was revealed that the 17-year-old suffers from a rare heart defect, Eileen McDonald said she was unhappy the information had been leaked.
Speaking after the revelation that Nicholas has Long QT Syndrome, his 45-year-old mother said: "We didn't want this to come out while Nicky was in the competition.
"We made it very clear to The X Factor that if this came out we would take him out of the competion. We wanted Nicky to move through the stages on his own merit and not progress because of a sympathy vote.
"We have always declined to talk about it. Nicky makes every day count and that's how we deal with it. He has already had to give up football to the condition and we didn't want the same thing to happen here."
Long QT is a serious heart condition that can cause fainting and episodes of an abnormally rapid heart rhythm, called arrhythmia, which can be fatal.
But, despite the illness, Nicky has raced through this year's X Factor, last night making it through the first part of the live final at Wembley to compete against Sam Bailey tonight for the crown. The winner receives a record contract and will support Beyonce on the UK leg of her forthcoming tour. As part of his performances last night he sang with Westlife star Shane Filan.
The teenager, a pupil at St Aiden's High School in Wishaw, has received the full backing of his school and community since the first auditions in April.
Headteacher Tony Rooney said it was honour for the school to have a pupil develop such a high profile.
Rooney said: "We are very excited to see Nicholas reach so far in the competition and everyone here is behind him. Certainly the phone bill has gone up dramatically in my house as we've been voting for him every week.
"He is a very down to earth young man and was always very popular at school but it's been great to watch his confidence grow on the show week by week.
"We always knew he had a great talent and he has sung a lot for our school - at assembly, in church and in our school rock band. In fact, when Nicholas won a guitar in a competition a few months ago for singing with the rock band he donated it to Yorkhill Sick Kids Hospital. He's that kind of boy."
School friend Lauren Muir added: "Watching Nicky on live TV every weekend is a surreal reminder of how much he has achieved in such a short space of time.
"Everyone is backing him all the way. In the school canteen, there is a massive "Vote Nicky" poster on the window. On almost every locker, wall, and door around the school there are "Team Nicky" posters, and many people are even wearing paper Nicky masks."
Neighbours in the cul-de-sac where McDonald lives with his parents and three siblings - Kevin, 18, Lewis, 13, and Antonia, eight, have been just as supportive.
Posters ordering "Vote Nicky" are in most windows and locals speak highly of the young man who has shot to sudden fame.
When McDonald was brought home for a visit to Motherwell this week hundreds of locals turned out to greet him.
Jacqueline Currie, 47, said McDonald is like an older brother to her seven-year-old twins, Niamh and Olivia, and echoed the sentiments of his head teacher, also saying the performer is "down to earth".
She added: "My daughters are his biggest fans. We watch the show every week and can't believe that Nicky is there. He has really raised it to a whole new level with his performances and we are extremely proud.
"When he came here on Tuesday there were hundreds of people out on the street - it was amazing.
"He is a very polite, quiet and down to earth young man but you can see his confidence has grown and his personality is shining through. I just hope he wins. He has to win."
Fellow Scottish reality star Jai McDowall, who won Britain's Got Talent in 2011, said he hopes McDonald has good people around him to help him through the coming weeks.
McDowall said: "For Britain's Got Talent we were well supported and we were only away from our families for a couple of weeks. The X Factor experience seems a lot more intense and you are away from your home in a false environment for a very long time.
"I would have struggled to cope if I had been as young as Nicholas so I hope he has people around him who are going to give him the right support. But I wish him good luck - he's done really well."
Georgia Strange, of ITV, said that Nicholas is on strict voice rest this weekend, while not performing, and that he is "exhausted".
She said: "Nicholas is exhausted from his home coming, he is on strict voice rest because he has been rehearsing so much and has had so many songs and lyrics to learn.
"He is in good spirits and feeling confident but he knows he really needs the public votes."