fetal age vs fertilization age

fetal age and fertilization age both are nouns.

Word	Noun	Adjective	Verb	Adverb
fetal age	Yes	No	No	No
fertilization age	Yes	No	No	No

As nouns, fetal age and fertilization age are synonyms defined as:

fetal age and fertilization age: the age of an embryo counting from the time of fertilization

Other synonyms of fetal age include gestational age.

fetal age (noun)	fertilization age (noun)
the age of an embryo counting from the time of fertilization	the age of an embryo counting from the time of fertilization