Crystal Peace stood before Erie County Judge Ernest DiSantis and entered a plea of "no contest" to charges of involuntary manslaughter and two counts of endangering the welfare of a child.
Her child, Nathan Rosier Jr. died in his crib last October. He weighed less than six pounds when authorities found him.
His twin brother was also malnourished, but survived.
Peace will be sentenced next month.
The child's father and grandfather also face charges in that death.