INDIANAPOLIS (AP) _ A pregnant 29-year-old woman facing prison for fatally poisoning her son could receive a lenient sentence if she is sterilized, a judge said.

''She has no need for any more children. I can't order this, but she could consider sterilization. It would be a mitigating circumstance,'' said Superior Court Judge Roy F. Jones at a hearing Wednesday on the medical status of Melody Baldwin.

Jones' proposal was called ''outrageous'' by the Indiana Civil Liberties Union.

Miss Baldwin, whose baby is due in September, pleaded guilty June 10 to child neglect. She could be sentenced to 20 years in prison for admitting she gave Joshua Baldwin, 4, a fatal dose of a depressant in May 1986.

Miss Baldwin's sentencing was postponed last week because of medical problems, including excessive vomiting. She was under close observation in a psychiatric ward after trying to remove an intravenous tube supplying her and the fetus with nutrition.

She was not in court Wednesday, but her lawyer, Michael J. Donahoe, said he would tell her about the sterilization proposal. Miss Baldwin has already agreed to put her unborn child up for adoption.

Timothy L. Bookwalter, a lawyer appointed by the judge to guard the fetus's interests, favors sterilization.

''We can't protect her every time she gets pregnant. She is a dangerous person and could kill this kid too,'' he said.

ICLU attorney Richard Waples said Jones' proposal was ''outrageous'' and an impermissible condition of sentencing.

''The U.S. Supreme Court struck down the sterilization of criminals in 1942,'' Waples said.

During Wednesday's hearing, psychiatrist Dr. Balwant S. Mallik said Miss Baldwin was sane but had a personality disorder and was suicidal.