A consultant psychiatrist said she had suffered an 'acute stress reaction' whilst giving birth.
But the court heard she was possessive with her boyfriend and appeared to keep secrets from him and in March last year she told him she had a miscarriage when he did not even realise she was pregnant.
Staff at her workplace also became concerned when she complained of stomach cramps and having an ectopic pregnancy.
Detective Chief Inspector Jill Johnston, of Lancashire Police, who led the investigation, said: 'This was a truly horrific incident involving the callous and brutal murder of a defenceless new-born baby.
I cannot begin to understand why Rachel Tunstill committed this horrendous act.'The person who should not be forgotten in this is the victim.
It emerged Mr Kelly had been led to believe Tunstill was just a few weeks pregnant and had no idea Mia had been born almost full term.