Letby came under investigation following a high number of unexpected infant deaths which occurred at the neonatal unit of the Countess of Chester Hospital three years after she began working there.

Since the conclusion of her trials and the lifting of reporting restrictions, experts have expressed doubts about the safety of her convictions due to contention over medical and statistical evidence, technical errors, and motive.

Medical professionals contested the interpreted diagnostics as "not sufficient" for criminal evidentiary use and "implausible", arguing the autopsies indicated the infants had died of natural causes.

[21][9] During a hospital visit in February 2016, the Care Quality Commission (CQC) was informed of difficulties in raising concerns with managers, but heard no mention of an elevated mortality rate.

The unit's services were scaled back by hospital managers on 7 July 2016, cutting cot space numbers and increasing the gestational age limit for admission from a minimum of 27 to 32 weeks.

[32][33] In March 2017, four consultants, including Stephen Brearey and Ravi Jayaram, asked management to involve the police after receiving advice for further investigation from the regional neonatal lead.

Sounds like my kind of case.”[41][42] During the police investigation which followed, Evans was instructed to review clinical records of the babies in the unit who had died or collapsed suddenly, in total 61 cases.

[80] Criminal psychologist David Holmes has argued that the varied methods she was said to have used to attack the infants, such as insulin and air injections and overfeeding milk, would all have been specifically chosen as things that would dissipate and not be easily detected afterwards.

[85] The defence argued that the notes were "the anguished outpouring of a young woman in fear and despair", written while Letby was dealing with employment issues including a grievance procedure with the NHS Trust.

[85] Searches of Letby's home found sensitive medical documents under her bed, including nursing handover sheets, resuscitation records, and blood gas readings.

[96] In a November 2024 interview by John Sweeney, Dewi Evans acknowledged the known presence of pseudomonas in the neonatal unit's water supplies, leading to several cases of pneumonia.

In closing, he stated, "there was a deep malevolence bordering on sadism [...] you [Letby] have no remorse [...] there are no mitigating factors [...] the offences are of sufficient severity to require a whole life order.

[118][119][117] In response, Alex Chalk, Secretary of State for Justice, wrote that the government will "look at options to change the law at the earliest opportunity" to compel defendants to attend their sentencing.

[120] On 30 August 2023, Prime Minister Rishi Sunak announced that the UK government would introduce legislation to Parliament that would compel convicted criminals to attend their sentencing hearings, by force if necessary, or face the prospect of more time in prison.

[140] All of these four grounds were refused by the Court, with the judges' subsequent written statement concluding that the trial had been "thoughtful, fair, comprehensive and correct" and that none of the four legal challenges advanced by Letby were "arguable", saying that the criteria for the admission of fresh evidence had not been met.

[9] Slater and Gordon, a law firm representing two of the victims' families, issued a statement calling for the inquiry to have the power to compel witnesses to participate, since a non-statutory hearing "must rely on the goodwill of those involved to share their testimony.

[185] The Parliamentary and Health Service Ombudsman Rob Behrens, called for radical change to NHS management in order to prevent future similar occurrences.

[200] Several neonatologists have described the claim that Letby murdered three infants by injecting air into their stomachs via a nasogastric tube variously as "nonsensical or 'rubbish', 'ridiculous', 'implausible' […] 'fantastical'" and "not practically feasible".

[141] Since this appeal attempt, Lee has published an updated version of his 1989 paper, which he argues further undermines the prosecution's case by showing that venous air embolism has never been documented to result in patchy skin discolouration.

[197] A joint report by two neonatologists working with Letby's legal team argues that Child O's liver was accidentally punctured by a doctor inserting a needle in the wrong place.

[2][205] A shift chart shown to the jury, ostensibly placing Letby uniquely at the scene of every suspicious event, has been criticised on the grounds that the criteria for inclusion of incidents were unclear and likely biased, rendering it misleading.

[2] Aviv also highlighted a previous investigation into the increased mortality on the unit that was carried out by the Royal College of Paediatrics and Child Health (RCPCH), and the hospital's response.

"[2] Due to reporting restrictions imposed as a result of Letby's impending retrial, the online version of the article was disabled for British readers,[210][211] a decision which was questioned in Parliament by the Conservative MP David Davis.

Commenting on the leaked report, professor of medical microbiology David Livermore argued that the bacterial outbreak is a simpler explanation for the observed spike in deaths on the unit than that Letby had murdered several infants.

[216] Cheshire Police reviewed the use of the false data and concluded that it was used in evidence relating to nine infants, but only played a central role in the case of Child K, who Letby was not convicted of harming at the first trial.

Svilena Dimitrova, an NHS consultant neonatologist, and Roger Norwich, a medico-legal expert, have made official complaints to the General Medical Council about Evans' evidence.

[219] BBC special correspondent Judith Moritz, one of only four reporters allowed in the courtroom, told The Sunday Times that the debate over the conviction is missing key aspects the jury saw during the trial.

[222][221] During Letby's 2022–2023 trial, the prosecution's expert witness Dewi Evans referred to a 1989 article written by Dr Shoo Lee[100] to diagnose air embolism as the mechanism of murder of the babies.

Dr Lee, the president of the Canadian Neonatal Foundation, was not called as an expert witness during Letby's trial and, living in Canada, did not hear about the use of his research until after the verdict.

[224] On 3 February 2025, Lee appeared in a press conference with Letby's barrister, Mark McDonald, the MP, David Davis, and neonatologist and former president of the Royal College of Paediatrics and Child Health, Neena Modi.