The decision by Judge Hiller B. Zobel in a case that stirred emotions on both sides of the Atlantic means that the 19-year-old will not have to serve 15 years in prison before becoming eligible for parole.
Sentencing was set for later today.
Manslaughter carries a maximum sentence of 20 years in state prison, but no minimum.
Ms. Woodward was convicted last month by a Middlesex Superior Court jury of second-degree murder in the death of Matthew Eappen last February. Such a conviction carries a mandatory term of life imprisonment with a 15-year minimum.
Matthew died Feb. 9, five days after he was taken to a hospital. Prosecutor said evidence of a 2 1/2-inch fracture to the back of the baby's head and bleeding behind Matthew's eyes indicated the baby's head had been slammed against a hard surface.
They said Ms. Woodward hated her job because it interfered with her social life. The baby's parents said they had to reprimand her about her late hours and what they considered lapses in her child care duties.
But the defense said the baby actually had been injured two to three weeks before Feb. 4, pointing to testimony that a clear fluid was found in the baby's skull. Ms. Woodward denied hurting the child, saying she only shook him a bit when she found him unresponsive in his crib after a bath. She denied telling police she may have been "a little rough" with Matthew.
The decision was to have been posted on the Internet at midmorning. The posting did not take place at the time it was expected, for reasons that were not immediately explained. The prosecutor's office confirmed that the judge had reduced the verdict.
Awaiting the decision this morning, reporters were asked to leave the courthouse. Ms. Woodward remained in the women's prison in Framingham, where court officials said she had access to a television.
Prosecutors had awaited the decision inside the court building. Defense attorneys were huddled in their offices, and reporters were barred from the building.
Ms. Woodward's supporters, who had been keeping vigil outside the courthouse, were asked by the judge to wait instead at a neighborhood restaurant, where a handful were gathered this morning around computer terminals.
Under Massachusetts law, Zobel had four options in deciding the appeal of the conviction: Let the verdict stand; dismiss the conviction; order a new trial; or reduce the finding to manslaughter.
Defense attorneys had conceded they made a mistake in their all-or-nothing strategy of limiting the jurors to consideration of acquittal or first- and second-degree murder, and not manslaughter.
But the prosecution said Ms. Woodward and her defense team were just looking for a better deal, and should not be allowed a reduced finding unless she admits what she did.