27-year-old Adrian Allen pleaded guilty Tuesday in court.
He was automatically sentenced to prison.
Allen was babysitting his girlfriend's three children at an apartment in the 2500 block of Knights Road in Bensalem on the weekend of February 24th to 26th.
Investigators say Allen swung the 18-month-old child around causing massive head trauma.
Police say an evaluation showed injuries consistent with child abuse including serious internal bleeding in her skull that required emergency surgery, evidence of blunt force trauma to her head, an adult-sized bite mark on her arm, broken ribs, an arm fracture and bruising all over her body.
The girl died a little more than a week after the assault.