noun, plural fe·tus·es. Embryology.
- (used chiefly of viviparous mammals) the young of an animal in the womb or egg, especially 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 plural -tuses
- the embryo of a mammal in the later stages of development, when it shows all the main recognizable features of the mature animal, esp a human embryo from the end of the second month of pregnancy until birthCompare embryo (def. 2)
late 14c., “the young while in the womb or egg,” from Latin fetus (often, incorrectly, foetus) “the bearing, bringing forth, or hatching of young,” from Latin base *fe- “to generate, bear,” also “to suck, suckle” (see fecund).
In Latin, fetus sometimes was transferred figuratively to the newborn creature itself, or used in a sense of “offspring, brood” (cf. Horace’s “Germania quos horrida parturit Fetus”), but this was not the basic meaning. Also used of plants, in the sense of “fruit, produce, shoot.” The spelling foetus is sometimes attempted as a learned Latinism, but it is not historic.
n. pl. fe•tus•es
- The unborn young of a viviparous vertebrate having a basic structural resemblance to the adult animal.
- In humans, the unborn young from the end of the eighth week after conception to the moment of birth.
- The unborn offspring of a mammal at the later stages of its development, especially a human from eight weeks after fertilization to its birth. In a fetus, all major body organs are present.
The embryo of an animal that bears its young alive (rather than laying eggs). In humans, the embryo is called a fetus after all major body structures have formed; this stage is reached about sixty days after fertilization.