Police officials said they were investigating the matter after the bodies of the four minor girls were found in a water tank in the Samungli area of Quetta, the provincial capital.
"Initially, their father Muhammad Khan said that his wife was mentally ill and had poisoned the minor girls," a police official said.
"But doctors at the hospital where the bodies were taken said they have been killed by drowning," he said.
The bodies of the girls - identified as Zakia, Farzana, Parwana, Yasmeen - were shifted to Quetta Civil Hospital for autopsy, where the mother was also admitted in an unconscious state.