The term foetus is an alternative spelling of fetus, which is the medical term for an unborn baby in the later stages of development. An EMBRYO is the medical term for an unborn baby in the earlier stages of development. Both terms refer to the unborn offspring of a human or other animal. Foetus and EMBRYO are often used interchangeably, but foetus typically refers to the later stages of development, while EMBRYO refers to the earlier stages.