fetus vs teras

fetus and teras both are nouns.

Word	Noun	Adjective	Verb	Adverb
fetus	Yes	No	No	No
teras	Yes	No	No	No

As nouns, teras is a hyponym of fetus; that is, teras is a word with a more specific, narrower meaning than fetus:

fetus: an unborn or unhatched vertebrate in the later stages of development showing the main recognizable features of the mature animal
teras: (medicine) a grossly malformed and usually nonviable fetus

Other hyponyms of fetus include monster, abortus, baby.

fetus (noun)	teras (noun)
an unborn or unhatched vertebrate in the later stages of development showing the main recognizable features of the mature animal	(medicine) a grossly malformed and usually nonviable fetus