Jamie-Lee Lauren Cooper, 30, of Mill Lane, Billinghay, originally faced a charge of child neglect following an incident involving the five month old.
She denied child neglect when she appeared before Lincoln Crown Court but admitted a further charge added to the indictment of inflicting grievous bodily harm arising from an incident on January 2, 2017.
Mark Watson, for Cooper, told the court that the offence his client had pleaded guilty to involved a single episode which was a momentary loss of control.
John McNally, for the prosecution, said that the plea of guilty to the new charge was “an acceptable compromise”.
Recorder Gareth Evans QC adjourned sentence for the preparation of a probation report.
Cooper was granted unconditional bail to appear back before the Crown Court for sentence on September 28.