Letby, 32, is said to have injected air into the bloodstreams of the infants while on duty at the Countess of Chester Hospital’s neo-natal unit.

The alleged injections are said to have caused air embolisms, in which gas bubbles enter a vessel or artery and can block the passage of blood.

On Tuesday, blood expert Professor Sally Kinsey told jurors at Manchester Crown Court she had been asked by Cheshire Police to review the cases of Child A, B and E.

The court has previously heard the mother of premature-born twins Child A and Child B had a rare auto-immune disease which led to an increased risk of blood clots.

However retired paediatric haematologist Prof Kinsey said the condition was not passed on to either her twin son or daughter.

Blood count results for both were also normal during their stay at the unit, the court was told.

Prof Kinsey said that reading the various descriptions of skin discolouration given by medical staff attending Child A’s fatal collapse “really cemented my concern”.

One doctor later told police about “flitting patches of pink areas on a background of blue/grey skin” which came and went in a pattern he had not seen before.

OTHER NEWS:

• Fraud-accused Herefordshire builder sent to crown court
• Herefordshire farmer banned after going to the pub with friends
• Hereford sex offender caught with thousands of child porn pictures

Nick Johnson KC asked: “Was the description consistent with a particular phenomenon?”

Prof Kinsey replied: “Seeing air embolism features in the skin.

“I have never seen it myself. I have seen it in medical literature but it was a pretty stark description of what sounded like air embolism to me.”

Child A died on the evening of June 8 2015 and it is alleged that his sister, Child B, who survived, was attacked by Letby on the following night-shift.

Prof Kinsey said she made similar observations about the medical staff’s comments on Child B’s skin discolouration of “purple blotchiness” during her collapse.

Jurors have previously heard that Child E, one of premature-born twin boys, lost up to a quarter of his blood volume, including some in his unsuccessful resuscitation in the early hours of August 4 2015.

Prof Kinsey said she noted a medic’s description of “purple discoloured patches” over Child E’s abdomen following the collapse which led to his death.

Ben Myers KC, defending, said: “Your evidence of (Child E) is that there was no explanation for spontaneous bleeding. What you mean is in terms of haematology, isn’t it?”

“Yes,” said Prof Kinsey.

Mr Myers: “That doesn’t mean that he might not have had a gastrointestinal haemorrhage or some other reason?”

Prof Kinsey said: “I agree.”

Mr Myers said: “And your assessment does not establish the cause of the bleeding. It simply establishes the cause was not a blood abnormality, is that right?”

“Yes,” said Prof Kinsey.

Mr Myers went on: “You have no expertise in dealing with air embolism, do you?”

Prof Kinsey replied: “No. Very few people have.”

The defendant, originally from Hereford, denies murdering seven babies and the attempted murders of 10 others between June 2015 and June 2016.

The trial will continue on Wednesday.

Want to stay up to date with all the latest Herefordshire court and crime news? Click here to join our Hereford Times Court and Crime group on Facebook or click here to sign up for our weekly court and crime email newsletter.