Sqad - I think it's best to use the term post natal psychosis here (as the judges and psychiatrist did) Post natal depression runs the spectrum from mild to serious depression and the most extreme ( and rarest) form is postnatal psychosis.
Psychosis is characterised by
hallucinations
delusions
confused and disturbed thoughts
a lack of insight and self-awareness
"A psychiatrist said her symptoms ‘fitted perfectly’ with a mental illness known as postpartum psychosis."
http://www.expressandstar.com/news/crime/2013/06/21/wolverhampton-rubbish-chute-baby-mother-cleared-of-attempted-murder/
The judge may have been swayed by the fact that the mother alleged that the child had been abducted.
It is a of course a tragedy that the infant suffered such horrendous injuries, and the mother will surely suffer from the knowledge that she inflicted them for the rest of her life.