A spokeswoman for the Children's Hospital in Boston confirmed that the child, Matthew Eappen, died shortly after 11pm last Sunday evening from injuries to the brain. The official cause of death was cited as "intracranial haemorrhage", or bleeding in the brain.
Ms Woodward, who enrolled with a nanny agency in the Boston area last June, had been working for the Eappen family since November. She arrived in the United States from her family home in Chester, Cheshire.
The drama will rekindle debate in the US about the wisdom of leaving children in the care of strangers as dramatised in the chilling hit film The Hand that Rocks the Cradle.
This real-life tragedy began last Tuesday when Ms Woodward telephoned the emergency services from the Eappen home asking for help and saying that Matthew was injured and had stopped breathing.
Suspicion that the child's condition had been induced by shaking was aroused on his admission to the hospital. Doctors found that his eyes were bulging, a common symptom of shaken-baby syndrome. They confirmed that the brain had been traumatised by repeated smashing against the side of the skull.
The teenager later told police that she had treated Matthew roughly in the family bathroom and had shaken him later because he had become fussy and she had become frustrated and lost her patience.
Matthew was kept alive on a life-support system from the moment of his admission last Tuesday until his death on Sunday.
Ms Woodward was jailed last week on initial charges of assault and battery. Bail was set at $100,000 (pounds 62,500). Her father, Gary Woodward, a builder, has flown to Boston but has not been able to raise the bail money. The bail is likely to be set higher, if murder charges are pressed.
In announcing the initial charges, prosecutors made it clear that they could be replaced by murder or lesser manslaughter charges if the boy was to die. Yesterday, district attorney Thomas Reilly said the feelings of the victim's family would be taken into consideration. "We have made no decision yet," he said. "Yes, there is the probability there will be additional charges. At this point I see no reason to rush."
Matthew's parents are both doctors in Boston. Sunil Eappen, 30, is an anaesthesiologist at Brigham and Women's Hospital. His wife, Deborah, 31, is an ophthalmologist. They have an older son, Brendan, who was also in Ms Woodward's care.
In a brief statement issued on their behalf by the hospital, the couple said: "Matthew was a tremendous gift and joy to us, and he will forever be part of our lives."
In England, Louise's mother, Sue Woodward, spoke yesterday about her own distress over the child's death. "Yesterday all the news had been positive because we hadn't heard anything about Matthew's condition," she said.
"That is all we were praying for," Mrs Woodward added. "I can't imagine how his parents must feel. I have already lost my daughter - but I have at least got the hope of getting her back".
Ms Woodward would normally face a trial in Massachusetts and could be imprisoned for life if found guilty. Her own life is not threatened, however, because the state, unlike a majority in the US, does not practice capital punishment.
The teenager was referred to the Eappens by a nanny agency based in Cambridge, just outside Boston, called EF Au Pair. Until November she had been working in the home of Harris Komishane in another Massachusetts town, Manchester-by-the-Sea. She left the first family after arguments about when she should be expected to come home in the evenings.
Mr Komishane stressed, however, that he never had any concerns about his family's welfare at Ms Woodward's hands. "There was nothing that would give us any idea that she would anything to harm the baby or anyone else's child," he said.
Sue Woodward similarly expressed bafflement over what could have triggered such an assault against Matthew by her daughter. "Louise isn't the sour- faced person in the papers," she insisted. "She loved life and she loved people. Out of all of us she had the nicest nature. If there was someone that irritated her and wasn't very nice she would be kid to them and try to find out why people didn't like them."