WPXI-TVreports police arrested Tanishia Fielder on Friday on charges of attempted homicide, aggravated assault, terroristic threats and other charges.
The baby was stabbed near the eye. The baby's condition and that of his father weren't known.
Police found a knife hidden beneath a garbage bin behind Fielder's apartment. Witnesses who tried to help the man and baby before Fielder chased them away told police she was wielding a knife that matched the one under the bin.
The baby's father told police he and Fielder had argued earlier in the day.