Real Dictionary

fetus in American English
an unborn offspring of a vertebrate animal that is still in the uterus or egg, esp. in its later stages and specif., in humans, from about the eighth week after conception until birth
noun: 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

fetus in British English
noun: 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 birth