Matthew Eappen's eye injuries showed extreme force had been used on him - equalling nine on a scale of severity from one to 10, eye specialist Lois Smith told the court.
Dr Smith was giving evidence in the Middlesex Superior Court in Cambridge, Massachusetts, as the trial of 19-year-old Ms Woodward, of Elton, near Chester, entered its second week.
Dr Smith, an ophthalmologist, told the prosecuting lawyer, Gerry Leone, that she had seen "many hundreds" of cases of accidental trauma in children. The type of haemorrhages found in nine-month-old Matthew's eyes was "very, very rarely" seen in accident cases - in fewer than 1 per cent.
"It was seen in a case of a child in a baby carriage that was hit by a truck, but never in what we call household accidental trauma," she said. "It is a very extreme amount of force that is required, such as being hit by a train or falling from five storeys."
Dr Smith said the injuries would have to be inflicted by a combination of severe shaking and impact. They were caused minutes or only up to an hour or two before the baby was admitted to Boston's Children's Hospital on 4 February. He died in a coma five days later.
Ms Woodward denies first degree murder, which carries a life sentence without parole.
The prosecution alleges that Ms Woodward shook the child and slammed his head against a hard surface in a rage because she was frustrated by his crying and fed up with working for Matthew's parents, Deborah and Sunil Eappen, of Newton, near Boston.
But the defence claims the baby's massive brain injuries could have arisen from an undetected skull fracture, which was suffered earlier and probably accidental.
Dr Smith said yesterday that she believed all Matthew's injuries were suffered at the same time. If the injuries had been present earlier, the baby would not have appeared normal, could not have been fed earlier on the day he was killed and would not have been able to cry normally.
If the injuries had been present earlier, Matthew would have had breathing problems and would have appeared comatose soon after he was injured.
Dr Smith agreed that folds in the retina associated with impact injuries had not been noted on a drawing by a doctor who examined the baby on his admission to hospital. She saw them when she examined the eyes after Matthew's death.
Barry Scheck, for the defence, asked her if it was a fundamental part of her testimony that the first doctor's drawing was wrong and that he had missed the folds and haemorrhages.
She replied: "Yes."
Mr Scheck asked: "If you are right about the mechanics of how these folds happened, that should have been in?"
She said: "He should have drawn it, yes."
Later she agreed with Mr Scheck that "the doctor just didn't draw what was there".
Mr Scheck asked her about the force of the impact the baby must have suffered and her comparison to that of a baby carriage being hit by a car or truck. Dr Smith said: "I said if it was just impact it would have to be that kind of force. With a combination it is different. You can get these injuries from shaking alone."
The case continues.