prematurely vs untimely

prematurely is not an adjective while untimely is an adjective.

Word	Noun	Adjective	Verb	Adverb
prematurely	No	No	No	Yes
untimely	No	Yes	No	Yes

As adverbs, prematurely and untimely are synonyms defined as:

prematurely (adjective)	untimely (adjective)
	uncommonly early or before the expected time
	badly timed

prematurely (adverb)	untimely (adverb)
too soon; in a premature manner	too soon; in a premature manner
(of childbirth) before the end of the normal period of gestation