The mother of a baby girl who drowned in a bath tub has pleaded not guilty to murder, on the grounds she was suffering a mental illness at the time, but guilty to manslaughter.

The six-month-old girl drowned in the bath tub of a North Strathfield home in November 2010 after months of obsessive worrying by her mother that the girl had dwarfism.

The woman, who cannot be identified for legal reasons, pleaded guilty to the manslaughter of her six-month-old daughter, who drowned in a bath tub. Credit:Peter Rae

The woman, who cannot be named for legal reasons, appeared in the NSW Supreme Court on Wednesday.

Crown prosecutor Mark Hobart, SC, accepted the manslaughter plea on the basis the woman was suffering from a substantial impairment, being a schizophrenic disorder, at the time.