Toni Mitchell has told the disability royal commission she was advised to have an abortion when an ultrasound indicated her son Joshy would have Down syndrome.

The mother of a child with Down syndrome has told the disability royal commission she was advised to have an abortion when screening indicated her unborn baby had the genetic condition.

The commission sitting in Sydney is hearing from several people with cognitive disabilities as well as parents, medical practitioners, experts, advocacy groups and government representatives.

Toni Mitchell, from Toowoomba, says she's had to continually justify the life of her now-19-year-old son Joshua who has complex medical needs and lives with Down syndrome and autism.

Ms Mitchell was advised to have an abortion when she had an ultrasound in 2000, she told the commission on Wednesday.

After being told she was having a boy, an older man came into the room and told Ms Mitchell her baby had a heart condition and she would likely miscarry within several weeks.

The man also said it was highly likely the baby would have Down syndrome, the commission heard.

While Ms Mitchell was still crying the man said, "Here's your appointment for a termination", she said, and he gave her a piece of paper.

That moment set the tone for the rest of Joshy's life, Ms Mitchell said.

"In that moment they completely disallowed his life. They said he wasn't worth living."

Ms Mitchell said she threw the paper in the bin.

"That was the moment I had to start justifying my son's right to live and to be treated and I had to start justifying his value to be alive," she said.

"For the whole rest of my pregnancy ... they kept saying, you don't understand what you're doing.

"You don't understand the immensity of bringing a child like this into the world, you don't understand what a burden this is going to be on you and on society and your family.

"They kept just judging us based on my decision to give him a chance at life."

After Joshua was born he tested negative for Hirschsprung disease - a congenital condition that causes problems with passing faeces.

In the following years, specialists chalked up his chronic constipation to him being "lazy" and having low muscle tone as a result of his Down syndrome, Ms Mitchell told the commission.

"He would be in so much pain he couldn't walk, he couldn't eat, he couldn't move because he was so full," she said.

Every six weeks, Ms Mitchell would take her son to hospital for a manual evacuation.

"(They would) send us home and never look any further," she said.

Joshua in 2014 underwent colorectal surgery when Ms Mitchell switched to private healthcare.

A contrast study had revealed such significant deformities that surgery was the only option.

"That's when they said 'Oh no this is Hirschsprung disease. He was misdiagnosed, they didn't do an extensive enough test when he was born'."