morning sickness vs nausea

morning sickness and nausea both are nouns.

Word	Noun	Adjective	Verb	Adverb
morning sickness	Yes	No	No	No
nausea	Yes	No	No	No

As nouns, nausea is a hypernym of morning sickness; that is, nausea is a word with a broader meaning than morning sickness:

morning sickness: nausea early in the day; a characteristic symptom in the early months of pregnancy
nausea: the state that precedes vomiting

Other hypernyms of morning sickness include sickness.

morning sickness (noun)	nausea (noun)
nausea early in the day; a characteristic symptom in the early months of pregnancy	disgust so strong it makes you feel sick
	the state that precedes vomiting