Dr Dewi Evans, a consultant paediatrician, told jurors that Summer Peace had developed the condition after her collapse.
The five-month-old was rushed to hospital on September 8, 2017, after she became unresponsive and struggled to breathe.
Dr Evans told Birmingham Crown Court there was no evidence of Summer having pre-existing pneumonia prior to her collapse.
He said he would expect a baby to show “clinical features” – a number of symptoms – which he hadn’t found, jurors were told.
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Initial symptoms, described as being “non-specific”, include a sniffly nose, a bit of a cough and possibly a fever which would worsen.
The baby would become “more miserable” as symptoms including an increased heart rate, breathing issues and nasal flaring develop.
Dr Evans said it would lead to respiratory distress – issues with breathing – and the baby turning blue due to cyanosis, the court heard.
He explained the symptoms would happen over a period of “several hours”, with the symptoms worsening throughout the day, a judge heard.
The consultant paediatrician said it would be the “most severe form” of pneumonia which would lead to a baby collapsing, jurors were told.
And a baby with the ability to cry is not something one would “associate or expect” with any respiratory infection due to the energy needed, Dr Evans said.
He explained to jurors that crying requires energy to create the noise and with pneumonia, all of that energy would be concentrated on “surviving” instead.
Dr Evans concluded Summer had “no symptoms of any sort of respiratory illness, let alone a severe one” prior to her collapse on the afternoon of September 8.
The professional said the collapse would not have been caused by pneumonia and concluded, in his opinion, that it was caused by head trauma, a judge heard.
Mr Michael Turner QC, defending Summer’s father Philip Peace in the trial, questioned Dr Evans to see if it was possible the condition could be pre-existing.
Mr Turner cited sounds heard by paramedics, described as noisy and bubbling, when they listened to Summer’s chest as possibly being evidence of the condition.
Dr Evans, in response to the cross-examination, said it could be explained through transmitted noises – noises from the throat – due to the baby’s size.
He added it could also be – in conjunction with the first explanation – could be food or liquid being aspirated into her lungs which causes aspiration pneumonia.
Philip Peace, 42, of Himley Road in Dudley, denies murder and manslaughter.
The trial continues.