untimely vs prematurely

untimely is an adjective but prematurely is not an adjective.

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

As adverbs, untimely and prematurely are synonyms defined as:

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

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