The parents were watching television at the time with their 97-pound (44-kg) American Staffordshire terrier-Great Dane mix and their 3-day-old boy, said Sergeant Tu Nguyen of the San Diego police child abuse unit.
"The mom coughed unexpectedly and it startled the dog, which bit the baby, causing traumatic injury," said Nguyen, whose unit is called in to investigate the death of any child.
"The parents were able to separate the dog from the baby and they rushed him to the hospital but they were not able to save him," he said.
Nguyen, who called the death a "tragic accident," declined to say where the baby was injured and said police are waiting for the Medical Examiner's report on the exact cause of death.
The dog was taken into custody by San Diego County Animal Services, officials said. Any dog that bites a person is placed in quarantine by animal control, said Dan DeSousa, deputy director of the agency.
DeSousa said the family can direct animal control to euthanise the dog or they can claim the dog at the end of a 10-day quarantine, but so far the agency has not received any instructions.
