The infant had been rescued, 20 minutes later, by a neighbour heard her cries. The child was found, under rubbish in bins near her home in Charlton, south-east London.
Louise Kamill, for the prosecution, told Inner London Crown Court that the woman had deliberately abandoned the baby on 16 June last year.
The court heard the woman was arrested and charged. Police said that her fingerprints were found on the bag.
Giving evidence yesterday, the woman, who cannot be named, broke down when she told the court she had no memory of abandoning the baby in a dustbin.
The hearing was adjourned until today.