Is it ensure, insure, or assure?
1785, from still (adj.) + birth (n.).
stillbirth still·birth (stĭl'bûrth')n.
The birth of a dead child or fetus.
A child or fetus dead at birth.
The birth of a fetus that has died; particularly, birth of a fetus that has died in the uterus at a stage in development when an infant could survive on its own if born healthy.