A foetus is a baby, or a young animal, still inside the mother, before it is born. In careful science talk, embryo is the very early stage, and foetus is the later stage, when the body parts are more clearly formed — in humans, that is often counted from about eight weeks. Though actually, a lot of people just say foetus, or baby, for the whole time before birth, and that is how you will hear it in ordinary talk. The spelling with oe is British; American English writes fetus. Taiwan hospitals say 胎兒; many local English textbooks use the American spelling fetus. After birth, the word no longer fits — then it is a baby.