A Perth couple has been jailed for failing to get medical help for their "horrifically" injured nine-week-old baby, who was found to have at least 35 bone fractures including breaks in every "long bone" of her 4 kilogram body.

Sarah Jane Mary Deal, 24, and her ex-partner, Hayden Shane Brooks, 25, were told by a GP in May 2016 they needed to urgently take their daughter to Joondalup Hospital because of concerns the baby was having difficulty breathing, was irritable, had sunken eyes and was failing to thrive.

But instead the couple went home and it was not until two weeks later that Deal finally sought medical help.

The child was in such a bad condition she was then taken by ambulance to Princess Margaret Hospital, where she was found to have 35 fractures in various stages of healing, including injuries to her ribs, legs and arms, as well as bruising to her ear and an injury to her eye.

No charges laid over baby's injuries

In separate interviews with police, Deal and Brooks said they had not taken the child to hospital earlier because they were concerned Deal would lose her single parent Centrelink payment if it was discovered they were living together.

Deal and Brooks both pleaded guilty in the District Court to a charge of engaging in conduct that may result in harm to the child — which relates to their failure to seek medical help for the baby.

The two were not charged with causing the baby's injuries because the court heard it could not be proven how the injuries were sustained and who was responsible for inflicting them.

However the court was told family members of both Deal and Brooks had seen both of them "behaving roughly" with the child, including instances where she was "slammed" down into their laps.

Another family member also claimed to have heard on one occasion the baby "screaming like she was getting murdered".

Prosecutor Sarah Kavanagh said the child must have been "in significant pain" that "would have been obvious to any parent", and both Deal and Brooks were as liable as each other, because they had each made "an independent decision" not to take the baby to hospital.

No doubt injuries inflicted deliberately: Judge

Deal's lawyer David Fort said his client no longer used drugs, had taken steps to rehabilitate herself, including undergoing counselling, taking parenting classes and seeing a psychologist.

Mr Fort also claimed the offence happened while Deal was "in the grips of an abusive and controlling relationship".

Brooks's lawyer Angus Hockton described his client as "young, naive and needy" saying he had been "thrown into a relationship too quickly" and had "to care for young life when he was not ready for it".

"This is not an offence of someone with total disregard for the law, this is an offence of someone not coping."

Judge Alan Troy said there was no doubt the vast majority, if not all, of the baby's "horrific" injuries were inflicted deliberately, but on the material before him he could not determine who was responsible.

He noted reports indicated the child's injuries had healed quite quickly, and she was now described as a happy and content toddler who was in good health.

But he added it was impossible to make any firm conclusions about the long-term effects the abuse may have.

Judge Troy described the failure to get medical help for the child as "a profound breach of trust" saying it was "inconceivable" the two were not aware she was suffering.

"You both must have known she was in severe pain, that she had been deliberately injured," he said.

He said the offences were so serious that only an immediate term of imprisonment was appropriate.

After taking into account their pleas of guilty and their steps towards rehabilitation, he sentenced Deal to 18 months in jail, while Brooks was given a 20-month term.

They will have to serve half the sentences before they are eligible for release on parole.