fetus (n.)

late 14c., "the young while in the womb or egg" (tending to mean vaguely the embryo in the later stage of development), from Latin fetus (often, incorrectly, foetus) "the bearing or hatching of young, a bringing forth, pregnancy, childbearing, offspring," from suffixed form of PIE root *dhe(i)- "to suck."

In Latin, fetus sometimes was transferred figuratively to the newborn creature itself, or used in a sense of "offspring, brood" (as in Horace's "Germania quos horrida parturit Fetus"), but this was not the basic meaning. It also was used of plants, in the sense of "fruit, produce, shoot," and figuratively as "growth, production." The spelling foetus is sometimes attempted as a learned Latinism, but it is not historic.