Letby, 33, was on duty at the Countess of Chester’s neo-natal unit when the blood oxygen levels of the premature-born infant unexpectedly dipped.
Giving evidence on Monday, February 13, consultant paediatrician Dr John Gibbs said two “profound” desaturations around 5am on November 27, 2015 were “alarmingly low”.
He told Manchester Crown Court that the youngster, Child J, was stable for weeks after she previously underwent bowel surgery a day after her birth.
Child J recovered quickly after she received breathing support, but more desaturations followed at 6.56 am, together with a marked fall in heart rate.
Dr Gibbs also noted all of Child J’s limbs were “stiffly extended with her hands clenched”.
He said the stiffness took 10 minutes to settle which was a “reasonably long seizure”.
More breathing support, via a facemask, led to an improvement – but a further similar episode took place at 7.24am, jurors were told.
Dr Gibbs, now retired, said: “Again, (Child J’s) oxygen and heart rate dropped and she needed support for her breathing just as in the first episode.
“This episode was much shorter. She did display the stiffness of all her limbs and clenches of her hands, and on the second episode her eyes deviated to the left. It stopped after three or four minutes.”
He said that Child J had not suffered a seizure either before or since.
Simon Driver, prosecuting, said: “You cannot be certain as to what was the cause?”
Dr Gibbs replied: “All the investigations did not reveal any causes for her seizures.”
Ben Myers KC, defending, said: “It’s not possible to say for sure whether the seizures caused breathing difficulties or breathing difficulties caused the seizures?”
Dr Gibbs said: “That’s correct.”
Mr Myers went on: “To be clear, you favour breathing difficulties first because your assessment was the drop in oxygen (levels) came before seizure?
Dr Gibbs said: “Yes. Exactly why and how the oxygen dropped, I don’t know.”
Fellow consultant Dr Stephen Brearey told the court: “She (Child J) normalised very quickly over the course of the day and all the investigations we had undertaken couldn’t identify why she had the desaturations or explain why she had a seizure.
“I do know hypoxia causes seizures so that would be possibly the most likely cause.
“There remains the question why was (Child J) hypoxic when two or three weeks beforehand she had been breathing normally in air and there was no suggestion of infection.
“In fact, we stopped antibiotics 36 hours after starting them because there was no evidence of infection in the blood tests and she remained well for weeks afterwards.”
Earlier, nurse Mary Griffith – who worked more than 40 years at the Countess of Chester before retiring in 2016 – agreed with Mr Myers that Letby was “knowledgeable”, “caring” and “thorough” in her job.
She also agreed there seemed to a “very high admission rate” to the neo-natal unit around 2015/2016.
Child J was discharged home in January 2016 and was described as progressing “very well” at her final outpatient clinic more than a year later.
Letby, originally from Hereford, denies murdering seven babies and attempting to murder 10 others between June 2015 and June 2016.