Is it ensure, insure, or assure?
"baby born prematurely," 1927, premy, American English shortening of premature + -y (2). Spelling with -ie attested from 1949.
preemie pree·mie or pre·mie (prē'mē)n. A prematurely born infant.
A premature baby: like a human preemie, it was placed in an Isolette (1927+)