have a bun in the oven vs birth

have a bun in the oven is not a noun while birth is a noun.

Word	Noun	Adjective	Verb	Adverb
have a bun in the oven	No	No	Yes	No
birth	Yes	No	Yes	No

As verbs, birth is a hypernym of have a bun in the oven; that is, birth is a word with a broader meaning than have a bun in the oven:

Other hypernyms of have a bun in the oven include bear, deliver, give birth, have.

have a bun in the oven (noun)	birth (noun)
	the event of being born
	a baby born; an offspring
	the process of giving birth
	the kinship relation of an offspring to the parents
	the time when something begins (especially life)

have a bun in the oven (verb)	birth (verb)
be pregnant with	cause to be born