A hole between the heart's two entry chambers, allowing freshly oxygenated blood to mix with oxygen-depleted blood - called an atrial-septal defect.
Cushla Ross says an ultrasound scan 20 weeks into her pregnancy detected an abnormality, but not the details.
Enough was found, however, to know that Riley should be delivered at Auckland City Hospital rather than as originally planned at a primary birthing unit. "If we had [done that] he wouldn't be here."
Riley was taken to the neonatal intensive care unit immediately after his birth.
Tests were done to diagnose the heart defects and he spent five days in Starship's high-dependency unit before his open-heart operation.
"He had an unusual anatomy regarding his veins and arteries so bypass was much more complicated," Mrs Ross says. "His chest was left open for more than 24 hours in case of swelling and was sutured up the next day."
The operation involved severing the aorta and suturing it back together without the narrowing, and patching the hole with some tissue taken from the sac surrounding the heart.
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Riley spent a week in Starship's intensive care unit after the operation and two weeks on a ward.
"He has made amazing progress," says his mother. "We've been very, very lucky.
"He's into everything - very into his rugby or any sport that he cando."
His under-7 Drury non-tackle "rippa" rugby team was undefeated this year and he came second in the Year 2 group in his school's cross-country run.
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Riley told the Herald his favourite activity was "playing rippa rugby".
Mrs Ross says the only time Riley's condition is noticeable is on the uphill sections of family hikes in the Hunua Ranges. "He just has to stop and catch his breath and have a rest and go again. On the rugby field he's absolutely fine."
"He pushes himself constantly and has had several broken bones and bruises from pushing the boundaries.
"He has never considered himself a heart child.
"Starship were our life-savers," Mrs Ross says. "They have told us Riley would not be alive without this life-saving surgery and their pre- and post-op care, which was incredible.
"They think they are just doing their job, but to us they are angels."
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