Mrs McDonnell found out when she was 24 weeks pregnant that her baby had a condition called intra-uterine growth restriction which meant he would have to be delivered prematurely by Caesarian section.
"It's not quite what you planned for a first baby," she says.
She went into hospital at 27 weeks and Archie was born at 30 weeks, weighing just 795 grams and measuring 34cm - about two hand-lengths.
He started life in an incubator, on constant oxygen support, and was fed intravenously for the first seven weeks.
"He couldn't breastfeed, he was too small," Mrs McDonnell says. "He did try. I pumped for five months so he was fed breast milk but just not breast-fed.
"We couldn't hold him for the first week. It was a very special moment when we could pick him up."
Instead, the McDonnells had to go back to their Ellerslie home each night without their baby.
"We went in first thing in the morning and stayed till 5 or 6 at night and were there with Archie as much as we possibly could, but you walk out at night and leave your baby in the care of those amazing people," Mrs McDonnell says.
"We just can't get over how amazing all the staff were - the nurses, the doctors, and also the receptionists, the cleaners, everyone in that whole unit is just incredible."
Mrs McDonnell, a teacher, and her husband, a senior project manager in information technology, knew about the Givealittle website and decided to use it so that raising money was "above board and open". They are discussing how to use the money with the unit's staff.
"We just want to give them something they can benefit from, whether it's food or treats, just something that is going to make them smile," Mrs McDonnell says.
The newborn intensive care unit cares for up to 46 babies at a time and about 900 babies a year, including 160 born weighing less than 1500 grams.
People can donate to the cause until the end of July at Archie's Givealittle page.
