(used chiefly of viviparous mammals) the young of an animal in the womb or egg, esp. in the later stages of development when the body structures are in the recognizable form of its kind, in humans after the end of the second month of gestation.

noun

blastosphere , blastula , developing infant , embryo , fertilized eggnotes:a human embryo is the earliest stage of development while a fetus is from the eighth week of development to birth