When Lucy Letby, a former nurse in a neonatal unit at a hospital in northern England, was found guilty last year of murdering seven babies and attempting to murder six others, Britain reacted with horror.

The prosecution told the jury that she had harmed babies through a macabre range of attacks: injecting them with air, overfeeding them with milk, infusing air into their gastrointestinal tracts and poisoning them with insulin.

Ms. Letby, 34, was handed 15 mandatory life sentences. But in the months since, a growing number of experts have suggested that the evidence used to convict her was flawed. Serious questions were first raised in a 13,000-word New Yorker article in May. Since then, dozens of statisticians and medical experts have expressed concerns.

Ms. Letby has always maintained her innocence. In May, her request to appeal her original murder convictions was denied. She was separately convicted in July in a retrial of one count of attempted murder, and on Thursday judges will consider her right to appeal that ruling.

Here’s what to know about the main concerns raised about Ms. Letby’s convictions.

Statistics were misused, many experts say.

After Ms. Letby was found guilty last year, Britain’s health secretary announced a public inquiry — an official investigation conducted by a judge, with hearings in public — to discover how a serial killer could get away with her crimes for so long. That began last month in Liverpool and is expected to continue until early next year.

But in July, 24 experts in statistics, forensic science and neonatology wrote to the British government, raising concerns over the way statistics and medical evidence were used in the case and calling for the inquiry to conduct “a broader examination of potential factors contributing to the increased neonatal deaths, without the presumption of criminal intent.”