pang vs birth pangs

pang and birth pangs both are nouns.

Word	Noun	Adjective	Verb	Adverb
pang	Yes	No	No	No
birth pangs	Yes	No	No	No

As nouns, birth pangs is a hyponym of pang; that is, birth pangs is a word with a more specific, narrower meaning than pang:

pang: a sharp spasm of pain
birth pangs: a regularly recurrent spasm of pain that is characteristic of childbirth

Other hyponyms of pang include labor pains, labour pains, afterpains.

pang (noun)	birth pangs (noun)
a sudden sharp feeling	a regularly recurrent spasm of pain that is characteristic of childbirth
a sharp spasm of pain
a mental pain or distress