Western Australian mum jailed for pouring bleach into her baby son's feeding tube

William was born with a rare congenital birth defect.

William was born with a rare congenital birth defect called Pierre Robin sequence, which left him with an undeveloped jaw and cleft palate that meant he needed to be fed through two tubes.

He was just 13 months old in December 2018 when his mother, Brooke Evelyn Lucas, poured about 30ml of bleach into his feeding tube.

William went to sleep then woke up vomiting and became seriously ill from the chemical.

But the poison would not have killed him, the Western Australian Supreme Court heard on Friday.

Lucas, who also has a daughter, was originally charged with attempting to murder William but pleaded guilty to the lesser charge of committing an act that was likely to endanger the life of her son.

The now 28-year-old was sentenced to two-and-a-half years in prison after Justice Joseph McGrath took into consideration her mental health issues.

"Your offending must be understood in light of your mental health issues and the circumstances that you have confronted since the birth of the victim," he said.

The court heard Lucas' actions were a cry for help, as she struggled to care for William, who needed around-the-clock care for his condition.

Justice McGrath said Lucas' actions were extremely dangerous.

"You did not want to or intend to endanger the life of your child, but rather your action was a cry for help from an overwhelmed mother with significant mental health issues," he said.

"You are a mother who was not coping with a son who has the most extraordinary, complex care needs.

"You reached a point where you did the unforgivable act of harming your own son as a cry for help."

Justice McGrath said her mental health condition impaired her functioning to such an extent as to reduce her moral culpability for the offending.

But he also noted the boy was entirely dependent on Lucas and others for his care and survival.

"The victim should have been protected and cared for by you," he said.

"There was some planning in your action, Ms Lucas, given that you had previously researched bleach poisoning on the internet.

"You had nearly three hours after placing the bleach in the tubes to reconsider your action, but you did not do so.

"You knew during that period that your own son was going to ingest the bleach."

Justice McGrath acknowledged that when William became unwell, Lucas called emergency services.

But she then emptied the contents of the feeding bag into the bathroom sink while ambulance officers attended to the baby.

She later blamed her daughter for putting the bleach into the infant's milk.

Eventually, she confessed what she had done.

The court heard Lucas no longer had contact with her children, who were now aged 6 and 3 years respectively.

"You have expressed feelings of grief from your loss of having contact with your children," Justice McGrath said.

He accepted that Lucas was remorseful and found she had good prospects for rehabilitation.

"I find that you are a person otherwise of good character, who committed the offence in the most extenuating circumstances," he said.

Lucas will be eligible for parole after serving 15 months behind bars.