Gemma Procter, 23, admitted killing 18-month-old Elliot, who fell from a sixth floor flat in Bradford.
She was originally charged with murder and due to go on trial at Bradford Crown Court today, but entered a plea of manslaughter by reason of diminished responsibility during a 10-minute hearing.
The plea was accepted and she will be sentenced on June 15, after psychiatric reports are compiled.
The incident took place at the Newcastle House tower block in Barkerend last October.
A previous court appearance in December was postponed after Procter's legal team said she was too ill to attend.