Thirteen rib fractures suffered by a four-month-old baby were deliberately inflicted, a court has heard.
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A medical expert said the injuries were consistent with the boy being squeezed, with his chest cavity compressed past one-third of its circumference.
The boy's father, who cannot be named to protect the child's identity, went on trial in the ACT Supreme Court on Wednesday accused of the abuse. He pleaded not guilty to two charges of assault occasioning actual bodily harm.
The baby, who is now in care, was taken to the Child Risk Assessment Unit at the Canberra Hospital for examination in May 2011.
Doctors found bruising on his abdomen, friction burns behind his ear, a superficial scratch under his chin and a small ulcer on his lip.
X-rays revealed 13 separate rib fractures, which paediatrician Judith Bragg said were consistent with a "deliberate inflicted injury".
Dr Bragg told the court the cause was likely compression, in an application of force consistent with squeezing or pushing.
She said the fractures were similar to those suffered in children who had had CPR.
The boy's mother, from the witness stand, said she saw the accused shake and squeeze the baby so hard he vomited and cried in pain.
She also told the court she saw the man punch the baby in the stomach and once barricaded himself in a room with the child.
But, under questioning by defence barrister Anthony Hopkins, the woman had trouble providing dates and time frames for the incidents.
When asked if she was inventing detail, she responded: "No, I'm telling the truth. He hurt [our baby]."
She said she had been scared to tell the truth at the time of the incident because the accused's mother had threatened to get custody of the boy and limit her contact.
In his opening address, prosecutor Anthony Williamson said the accused had questioned the boy's paternity, believing the mother's ex-boyfriend was the actual father.
Mr Williamson alleged the defendant was resentful and angry at raising a child he believed was not his.
But Mr Hopkins said the case turned on the credibility of the mother.
He said suspicion had initially fallen upon both parents and other potential sources, such as childcare workers.
But he said the mother had denied knowledge of the injuries for 18 months until she suddenly "pointed the finger" at the defendant.
Mr Hopkins said the motive for blaming his client was to absolve herself of wrong doing and to have the boy returned to her care.
He said the mother's accusations were disjointed and had grown increasingly exaggerated over time.
The judge-only trial before Justice Richard Refshauge continues on Thursday.