Officers were called to a property in New Road, Pontyberem, in Carmarthenshire, around 8.30am this morning.
A Dyfed-Powys Police spokesman said: "A police investigation is underway following the sudden death of an infant at a property in Pontyberem.
"Inquiries are still at an early stage with the call being received shortly before 8.30am.
"Further updates will be made available as appropriate."
The family dog has been seized as part of the inquiry.
Early, and as yet unconfirmed, reports suggest the animal is an Alaskan Malamute, which is not a banned breed.
However, police said the breed and type of dog seized is not being released at this stage.
