illegitimate child vs by-blow

illegitimate child and by-blow both are nouns.

Word	Noun	Adjective	Verb	Adverb
illegitimate child	Yes	No	No	No
by-blow	Yes	No	No	No

As nouns, illegitimate child and by-blow are synonyms defined as:

illegitimate child and by-blow: the illegitimate offspring of unmarried parents

Other synonyms of illegitimate child include bastard, illegitimate, love child, whoreson.

illegitimate child (noun)	by-blow (noun)
the illegitimate offspring of unmarried parents	the illegitimate offspring of unmarried parents