Three judges in London held that the mere presence of three classic features of "shaken baby syndrome" did not automatically lead to a conclusion of unlawfully killing or injury.
The court allowed an appeal by Raymond Rock, of Great Yarmouth, Norfolk, against his conviction of murdering his girlfriend's daughter Heidi Davis, aged 13 months, in 1998, and reduced the verdict to manslaughter.
He was alleged to have shaken the child violently, but he insisted she wriggled out of his arms and fell to the floor. He was still serving his sentence.
Lorraine Harris, 36, of Long Eaton, Derbyshire, who was jailed for the manslaughter of her four-month-old son Patrick McGuire in 2000, had her conviction quashed.
She said the baby became ill and stopped breathing after a vaccination. He had a blood disorder which was discovered only after his death.
She has been released from prison but was banned from seeing her other child.
Michael Faulder, 34, jailed for two-and-a-half years at Teesside Crown Court in 1999 for causing grievous bodily harm to a boy aged seven weeks, was also cleared.
He said he accidentally dropped the baby while trying to put him in his pushchair. The child made a full recovery.
But the judges dismissed an appeal by Alan Cherry against his conviction at Birmingham Crown Court in 1995 of the manslaughter of his girlfriend's 22-month-old daughter Sarah Eburne-Day.
He denied shaking her in a fit of temper and claimed she fell off a stool on which she was standing. He was no longer in custody.
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