stillbirth vs spontaneous abortion

stillbirth and spontaneous abortion both are nouns.

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

As nouns, stillbirth and spontaneous abortion are synonyms defined as:

stillbirth and spontaneous abortion: a natural loss of the products of conception

Other synonyms of stillbirth include miscarriage.

stillbirth (noun)	spontaneous abortion (noun)
a natural loss of the products of conception	a natural loss of the products of conception