Dr Irene Scheinberg gave evidence at Birmingham Crown Court into the death of five-month-old Summer who was allegedly killed by her father Philip Peace.
Prosecutors allege he shook Summer before she collapsed and suffered fatal brain injuries on September 9, 2017.
Peace, aged 42, of Himley Road, Dudley, is accused of her murder which he has denied.
More from the trial:
In cross-examination, Peace’s defence counsel, Michael Turner, asked Dr Scheinberg: “I would like your opinion as to whether this was pre-existing pneumonia or post-collapse pneumonia.”
She replied: “I think she had an infection prior to her collapse.”
Dr Scheinberg was asked about the same subject by the prosecution, Andrew Smith QC, to which she replied: “I think it is a strong possibility she had pneumonia before she collapsed.”
Mr Smith QC asked Dr Scheinberg if she could say what Summer’s cause of death was, from a paediatric point of view.
She told jurors: “No, I can’t say. I think 60 per cent she had pre-existing pneumonia, 40 per cent I have not got a clue.”
In response, Mr Smith said: “It is not that you do not have a clue, the other is traumatic brain injury.”
Alongside suffering a traumatic brain injury, Summer also sustained 11 rib fractures.
Her father administered chest compressions after Summer collapsed.
Dr Scheinberg was asked about medical evidence concerning Summer’s fractured ribs.
She said she did not accept all of those were fractures.
She appeared as a witness for the defence.
The trial continues.