Lucy Letby: How the case of one baby death shows flaws in the medical evidence on both sides

When it comes to the Lucy Letby case, there are two parallel universes. In one, the question of her guilt is settled. She is a monster who murdered seven babies and attempted to murder seven more while she was a nurse at the Countess of Chester Hospital between 2015 and 2016.

In the other universe, Letby is the victim of a flawed criminal justice system in which unreliable medical evidence was used to condemn and imprison an innocent woman.

This is what Letbys barrister Mark McDonald argues. He says he has the backing of a panel of the best experts in the world who say there is no evidence any babies were deliberately harmed.

These extremes are both disturbing and bewildering. One of them is wrong - but which? Who should we believe?

The families of the infants say there is no doubt. Letby was convicted after a 10-month trial by a jury that had considered a vast range of evidence. They say Letbys defenders are picking on small bits of evidence out of context and that the constant questioning of her guilt is deeply distressing.

I have spent almost three years investigating the Letby case - in that time I have made three Panorama documentaries and cowritten a book on the subject. Yet, if true, the new evidence, presented by Mark McDonald in a series of high-profile press conferences and media releases, is shocking.

According to his experts, the prosecution expert medical case is unreliable.

Mark McDonald has not released the panels full reports, which are currently with the Criminal Cases Review Commission (CCRC), the body he needs to persuade to reopen Letbys case, but he has released summaries of the panels findings.

Letby was found guilty of 15 counts of murder and attempted murder, and the jury in her original trial reached unanimous verdicts on three of those cases. That is a good indication of where the strongest medical evidence might lie.

To get a sense of the imperfections woven through both the prosecution and the defence arguments, its worth looking at one of those cases in which the guilty verdict was unanimous: that of Baby O.

Baby O was born in June 2016, one of triplet brothers. At Letbys trial, the jury was told that his death was in part the result of liver injuries, which the prosecution pathologist described as impact-type injuries - similar to those in a car accident.

As in other cases for which Letby was convicted, the prosecution said circumstantial evidence also tied her to the crime.

However, a paediatric pathologist who was not involved in the case but has seen Baby Os post-mortem report, says it was "unlikely" Baby Os liver injuries were caused by impact - as the prosecution claims.

"You cant completely rule out the possibility," says the pathologist, who does not want to be identified. "But in my view, the location of the injuries and the condition of the liver tissue itself dont fit with that explanation."

Which raises the obvious question - if the prosecution were wrong about Baby Os liver injuries, then why did he die?

Letby was accused of injecting air into the blood of Baby O as well as that of other babies. This, the prosecution said, caused an air bubble and a blockage in the circulation known as air embolism.

During the trial the prosecution pointed to several pieces of evidence to make their case, including a 1989 academic study of air embolism in pre-term babies, which noted skin discolouration as one possible feature of it.

Prosecutors argued that these same skin colour changes were observed in several babies in the Letby case.

However, Dr Shoo Lee, a Canadian neonatologist and one of the authors of that 1989 study, is now part of Letbys team of defence experts working with Mark McDonald. He argues that his study was misused.

He says skin discolouration has not featured in any reported cases of air embolism in babies where the air has entered the circulation via a vein – which is what the prosecution alleged happened in the Letby case.

In other words, the prosecution was wrong to use skin discolouration as evidence of air embolism.

It sounds significant. But is it enough to defeat the air embolism allegations?

As with many aspects of the Letby case, the answer is not clear-cut.

The prosecution did not rely on skin discolouration alone to make their case for air embolism. And although there have not been any reported cases of skin discolouration in babies where air has entered the circulation via a vein, some critics have argued that the number of reported air embolism cases is small and that the theory is still possible.

To muddy the waters further, another of Mark McDonalds panel of experts has said that in fact there was post-mortem evidence of air embolism in the babies.

"We know these babies suffered air embolism because of the post-mortem imaging in some of them," says Neena Modi, a professor of neonatal medicine.

She believes this is highly likely to have occurred during resuscitation, and that there are much more plausible explanations for the collapses and deaths of the babies in the Letby case than air embolism.

The air embolism theory, she said, was "highly speculative". But her remarks show the debate is far from settled.

There has been another explanation for Baby Os death.

In December 2024, Mark McDonald called a press conference in which one of his experts, Dr Richard Taylor, claimed that a doctor had accidentally pierced the babys liver with a needle during resuscitation. This, he argued, had led to the babys death.

Dr Taylor added: "I think the doctor knows who they are. I have to say from a personal point of view that if this had happened to me, Id be unable to sleep at night knowing that what I had done had led to the death of a baby, and now there is a nurse in jail, convicted of murder."

The doctor accused of causing the babys death was subsequently identified as Stephen Brearey – one of Letbys principal accusers at the Countess of Chester Hospital.

Mr Brearey says: "Given the ongoing investigations and inquiries, and to respect the confidentiality of those involved, I will not be making any further comment at this time."

It was a bombshell claim. But does the evidence support it?

One indication that the needle theory might be shaky was that Dr Taylor, by his own admission, had not seen Baby Os medical notes and was relying on a report that had been written by two other experts.

Another obvious problem with the needle theory is that it had already been examined at length during Letbys trial.

The prosecution pathologist concluded that there was no evidence that a needle had pierced Baby Os liver while he was alive and the paediatric pathologist we spoke to agrees.

They told us: "These injuries werent caused by a needle. They were in different parts of the liver and there was no sign of any needle injury on the liver."

Even if the needle had penetrated the babys liver, it cannot explain why Baby O collapsed in the first place or why he died - the needle was inserted after the babys final and fatal collapse towards the end of the resuscitation.

When asked if he still stood by his comments about the doctors needle, Dr Taylor told us that while the needle may not have been the primary cause of death, his "opinion has not substantially changed".

He said the "needle probably penetrated the liver" of Baby O, and "probably accelerated his demise".

The question of where this leaves the case presented by Mark McDonalds panel of experts when it comes to the needle theory is a difficult one to answer.

It would appear that among Letbys defenders, there is not consensus.

Consultant neonatologist Dr Neil Aiton is one of the authors of the original report on which Dr Taylor based his comments. Dr Aiton says that he has examined the evidence independently and has concluded that Baby Os liver injuries were caused by inappropriate resuscitation attempts, including hyperinflation of the babys lungs.

However, he also says it was "pretty clear" a needle had punctured the liver during resuscitation.

When Dr Aiton was told that other experts, including the paediatric pathologist who spoke to the BBC, have examined the case of Baby O and said that it is implausible to conclude this happened, he said that there were two possibilities. Either the liver ruptured because of a needle or it ruptured spontaneously.

Dr Aitons position appears to be that poor resuscitation caused the babys liver injuries and whether it was a needle or not is "not important".

That is a contrast from what Dr Taylor said in that December press conference. And critics say Dr Aitons account still does not explain why Baby O collapsed in the first place and why he needed such desperate resuscitation.

A summary report from Letbys expert panel appears to back further away from the needle theory. It says a needle "may have" punctured the liver.

Other experts, including the paediatric pathologist, said that Dr Aitons observation of hyper-inflated lungs would not explain Baby Os liver injuries.

Once again, the case illustrates how difficult it is to distinguish between plausible and implausible claims.

Since that press conference, other experts working for Letbys defence team have put forward another theory for Baby Os death. They say his liver injuries were the result of traumatic delivery at the time of birth.

Professor Modi says this was a "highly plausible cause".

But that has been contested from a surprising direction. Dr Mike Hall, a neonatologist, was Lucy Letbys original defence expert and attended court throughout her trial.

He has been a staunch critic of her conviction, arguing her trial wasnt fair and that there is no definitive medical evidence that babies were deliberately harmed.

However, Dr Halls view is that evidence for the birth trauma theory is simply not there. He notes that Baby O was born in good condition by caesarean section and there is no record of a traumatic delivery in the babys medical notes.

"Theres still no evidence that anyone did anything deliberately to harm Baby O," he adds. "However, something was going on with Baby O, which we havent explained.

"We dont know what the cause of this is. But that doesnt mean that we therefore have to pretend that we know."

For the jury, Baby O was one of the clearest cases that proved Letby was a killer. And yet there appears to be flawed expert evidence on both sides.

There were two other cases where the jury returned unanimous verdicts – the cases of Babies F and L.

The prosecution argued that both babies had been poisoned with insulin and highlighted blood tests that it said were clear evidence of this. For the prosecution, the insulin cases proved that someone at the Countess of Chester Hospital was harming babies.

Letbys defence have, meanwhile, marshalled numerous arguments against the insulin theory. One is that the blood test used - an immunoassay - is inaccurate and should have been verified. But even Letbys experts accept the test is accurate around 98% of the time.

Another argument is that premature babies can process insulin differently and that the blood test results are "within the expected range for pre-term infants". But the medical specialists weve spoken to are baffled by this claim and say it goes against mainstream scientific understanding.

Of course, mainstream opinion can be wrong. But it is difficult to tell because Letbys defence team have not shared the scientific evidence.

One of the experts behind the report – a mechanical engineer who carries out biomedical research – clarified that his analysis says the blood test results were "not uncommon". However, Letbys defence declined to show the BBC the published studies that support this claim.

Once again, the claims of both the prosecution and defence are not clear-cut.

Ultimately, the question of whether Letbys case should be re-examined by the Court of Appeal now lies with CCRC. They have the task of studying Mark McDonalds expert reports.

If he is successful and Lucy Letbys case is referred back to the Court of Appeal - that is ultimately where the expert evidence on both sides will face a true reckoning.

Lead image credit: Cheshire Constabulary, PA

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