The Maui dolphin that washed up on a beach north of Raglan earlier this week probably died from blood poisoning.
Department of Conservation said veterinary pathologists at Massey University established the dolphin had been pregnant, and that the death of the unborn calf several days before had caused the poisoning.
A marine adviser Kristina Hillock said the pathologists could not tell from a necropsy exactly why the fetus died, but the mother had been unable to give birth to it.
Remains will be delivered to local iwi once all testing is complete.