A woman suffering from severe postnatal depression killed her baby after her medication was taken away and not replaced, a court has heard.
Katy Norris of Brixham, Devon, admitted infanticide via a video link from a psychiatric unit in Milton Keynes.
The 31-year-old was sent to the secure unit in April - shortly after smothering her 10-day-old son Leo.
At Exeter Crown Court, the judge Mr Justice Jack made a Section 37 Hospital Order under the Mental Health Act.
At an earlier hearing, he described it as a "very sad and tragic case".
A serious case review into Leo's death is being carried out, the court was told.
Defence barrister Adam Vaitilingam QC said Mrs Norris was diagnosed with severe postnatal depression days before the baby's death.
"Her previous medication was withdrawn and no further medication was prescribed.
"One can understand serious questions are being asked by those close to her as to why more was not done."
Prosecutor Sarah Munro QC told the court that during her pregnancy Mrs Norris was prescribed anti-depression medication.
Following the birth of Leo in April she was initially happy but "deteriorated considerably" and the medical services became involved, the court heard.
On the morning of 20 April she killed Leo.
She was originally charged with murder, but a lesser charge of infanticide was agreed.
Mrs Norris admitted a charge of smothering a child under 12 months of age with a cushion while the balance of her mind was disturbed.
The judge said: "I am satisfied by the evidence I have read in reports that Mrs Norris is suffering from a mental disorder."
After the case, a spokesman for Torbay Care Trust confirmed a serious case review had been convened, but said details of an individual's care could not be discussed.