According to the police, the 40-year-old mother, identified as Suma, had been under psychiatric treatment for the past few months.
She reportedly turned up on her own at the women's cell of the Town police station in Alappuzha and confessed that she killed the child when her husband John was at work.
When she planned to commit the crime, Suma told her elder daughter to go to her grand parent's home after the school hours as she would be spending some time in the church.
The police recovered the body of the child floating on the water in the washing machine.
The woman would be produced before the magistrate on Thursday, Sep 23.