The child was found at a house in Bicester, Oxfordshire, on Thursday morning.
Thames Valley Police officers were called to the address in Herald Way, Glory Farm, after a report received at 10.01am concerning fear for the welfare of a child from a member of her family.
Police said it was an "unexplained death".
Senior investigating officer, Detective Chief Inspector Ian Hunter, said: "We believe the child was in the care of her mother at the time of her death and officers are currently trying to trace her whereabouts. At this time she is thought to have left the United Kingdom.
"We need to speak to the mother to establish the circumstances prior to the discovery of the child's body."
A post-mortem examination is being carried out tonight. A formal identification is yet to be done.
Police are expected to continue their watch at the scene overnight.
Superintendent Colin Paine, Local Policing Area Commander for Cherwell and West Oxfordshire said: "Incidents of this nature are extremely sad and will inevitably be felt throughout the community.
"I would like to reassure the local community that there is no risk posed to them and the local neighbourhood team will be carrying out patrols to speak to residents about any concerns they may have."