imminent abortion vs stillbirth

imminent abortion and stillbirth both are nouns.

Word	Noun	Adjective	Verb	Adverb
imminent abortion	Yes	No	No	No
stillbirth	Yes	No	No	No

As nouns, stillbirth is a hypernym of imminent abortion; that is, stillbirth is a word with a broader meaning than imminent abortion:

imminent abortion: the appearance of symptoms that signal the impending loss of the products of conception
stillbirth: a natural loss of the products of conception

Other hypernyms of imminent abortion include miscarriage, spontaneous abortion.

imminent abortion (noun)	stillbirth (noun)
the appearance of symptoms that signal the impending loss of the products of conception	a natural loss of the products of conception