A mother poisoned her weeks-old daughter using a powerful adult pain killer never used on babies, a jury has been told.
Michelle Smith, 34, is accused of the seemingly motiveless murder of 42-day-old Amy Smith at home in Morriston, Swansea.
The baby girl, who was seen by a health visitor on the day she died on 9 November 2007, was described as "thriving".
Within hours, paramedics urgently called to the address were forced to give up trying to resuscitate Amy's blue-lipped, lifeless body.
A blood test more than two weeks before her death found the powerful painkiller dihydrocodiene, Swansea Crown Court heard.
But the vital test result was never passed to her doctors and a defenceless Amy lived to suffer at least one more dose of the drug.
"To put it as neutrally as possible, it was unfortunate it did not come to the attention of the doctors in the case," said prosecutor Christopher Clee QC, describing the failure.
"The finding of dihydrocodeine was not only highly unusual but also highly significant," he said.
Side effects of the drug include drowsiness, a slowing-down of breathing and cardiovascular collapse which can lead to coma, the jury heard.
Both Smith and her husband Christopher Smith had previously been prescribed medication that contained the drug.
On the day of the death, Smith was home with two other young daughters and was the sole carer of all three.
She claims on the day Amy was described as "thriving" she took her upstairs to bed in the afternoon and discovered her dead one hour later.
As she was carrying what she described as her dead daughter downstairs, her husband came in from work.
Smith denies a charge of murder and an alternative charge of causing or allowing the death of the child.
When she was 17 days old, Smith took Amy to the A and E unit at Singleton Hospital, Swansea, claiming she found her "lying on her back perfectly still with her eyes wide open".
"She said that she had picked Amy up and she had felt floppy," Mr Clee said.
On examination, doctors found Amy's breathing rate was 20 per minute, half the normal rate, and she was making a "very poor respiratory effort".
She was given oxygen and prescribed antibiotics in case she had an infection.
Her condition worsened and she was put on a ventilator and transferred to the paediatric intensive care unit at University Hospital of Wales, Cardiff.
Her improvement there was rapid and she was transferred to a normal ward and eventually returned to Singleton Hospital.
Within days she was discharged after an examination which found her "well, alert and hungry".
Over the following three days Smith returned to hospital so Amy could receive antibiotics.
On the final visit, Smith told doctors that Amy was sleepy and had managed only a small feed.
Her breath ratio was then found to be as low as 10 per minute and she was put on a ventilator again and transferred back to Cardiff.
Doctors also sent a urine sample for analysis which screened positive for opiates. As a result it was sent away for more specialised analysis to a lab at the Princess of Wales Hospital, in Bridgend.
That test found dihydrocodeine in Amy's urine several weeks before she died, but the results were never passed on to doctors.
The trial continues.