A 13-month-old baby was found decapitated, while her seriously injured mother died on the way to hospital in Uttar Pradesh's Bulandshahr district, police said on Thursday.
The incident took place in Vijay Naglia village on the intervening night of Wednesday and Thursday, they said.
Senior Superintendent of Police (SSP) Santosh Kumar Singh said the woman, Jitendri (23), was mentally challenged and her husband is a tailor in Rajasthan.
On the night of the incident, Jitendri, her baby and her three sisters-in-law were sleeping in the same room while her one brother-in-law and his wife were sleeping in another room. The father-in-law of Jitendri and her other brother-in-law were sleeping outside the home, police said.
Around 1 am, the sisters-in-law of Jitendri heard someone jump off the roof. When they did not see Jitendri and her son in the room, they reached upstairs, where they found the baby decapitated, they said.
A sickle, a packet of poisonous substance and a milk bottle were also found, police said.
They along with the villagers, who arrived there upon hearing their screams, started a search for Jitendri. They found a room locked from the inside in a forlorn house on the backyard of their home, police said.
When they broke open the room, Jitendri was found lying unconscious with injury marks around the neck, they said.
Jitendri succumbed to the injuries on the way to hospital, police said.