Jessica Wiggins had been on trial following the death of 12-week-old Isabella Valance.
The 19-year-old, of Heathfield Road in Redditch, was acquitted of a separate count of child cruelty on Thursday.
Isabella's father, Tyler Vallance, 21, admitted unlawful killing before the trial began and is awaiting sentencing.
Prosecutors said they would seek a retrial in the case of Ms Wiggins.
Isabella died of a head injury in hospital five days after being found apparently lifeless when paramedics were called to her parents' flat in Stanley Close, Redditch, Worcestershire.
The court heard Vallance carried out two attacks that left Isabella with nine broken ribs.
A jury has failed to reach a verdict in the case of a mother accused of causing or allowing the death of her baby daughter.