Twins who were branded as 'lazy' actually have a critical muscle condition.
When Raffa and Sibby Whyman were unable to lift their heads and didn't have much movement, their parents Jenna and Steve became concerned.
Health workers allegedly assured the worried parents their tots would 'soon catch up', the Mirror reports.
Jenna, 36, and Steve spend months raising concerns with medics before being told something was wrong.
One day before their first birthday Raffa and Sibby were diagnosed with Spinal Muscular Atrophy (SMA) type 1.
The adorable tots would not likely live past age two if the condition was left untreated.
Jenna said she first became worried when the boys showed no signs of being able to lift their heads or roll over, but was repeatedly told not to worry.
As a result Raffa and Sibby lost vital time in their battle against the progressive condition, which causes muscle wasting and loss of movement.
She told The Mirror: "They had a rattly chest from birth, which we now know is a sign of SMA.
"Each time I'd raise that they're not attempting to lift their head or roll over I was told 'don't worry, they're twins, they'll catch up in their own time.
"'They're boys, they're probably just lazy.'
"Now they're unable to sit up and hold their heads up, while other children their age are starting to get on their feet and take their first steps."
As time passed and the boys showed no sign of improvement, swimming instructor Jenna and warehouse manager Steve, from Benfleet in Essex, pushed for answers.
On one occasion, Jenna said, she was branded "neurotic" as she questioned why her sons were developing at a slower rate than other children their age.
After a health visitor finally agreed that something was wrong, the twins were rushed for urgent tests - leading to the heartbreaking news they both had SMA1.
"I was googling and it kept coming up with cerebral palsy, up to that point I'd never heard of SMA," Jenna said.
"We were given a leaflet which said that babies with SMA type one don't usually live beyond their second birthday. It was a complete shock, we were so emotional.
"I'm normally a positive person but I'm only human."
They were referred to Great Ormond Street Hospital in London, where they learned of revolutionary treatments which could make a difference.
"Historically you'd be told to take them home and love them, and they wouldn't be here after two years," Jenna said.
"Even now with the treatment they're on, there's no data beyond the age of six, everything in their future is unknown."
The boys have undergone gene therapy using a new drug named Zolgensma, branded the world's most expensive drug because of its £1.7 million list price per dose.
The twins require specialist cough assist equipment to help them clear their lungs - something children with SMA1 are otherwise unable to do.
It is likely both Raffa and Sibby, who turned one in April, will require wheelchairs for life.
Jenna and Steve say that although the NHS care they have received since diagnosis has been "incredible", they are struggling to meet the costs of physiotherapy and hydrotherapy and specialist equipment.
Their home also requires extensive modification to make it accessible for the twins, and they have launched a fundraising appeal - which has already collected more than £13,500.
"It's so overwhelming, as a family we're so incredibly touched."
She said she hopes that by sharing her story, others in a similar situation will push for a diagnosis when they know something is wrong.
In the US all children are screened for SMA, but Jenna said other parents of children with the condition have had to battle to find out what is wrong.
"I just hope that one person reads my story whose child is facing an SMA diagnosis and they push the doctors to take them seriously," she said.
"If it saves just one child it'll be worth it."
To support the family's fundraising appeal, click here
