Developing human foetus. Illustration of a foetus in the early stages (8 weeks) of development. The developing baby is an embryo from just after conception to around 8 weeks. It is then known as a foetus from around 8 weeks until birth. The foetus at this stage is around 30 millimetres long and weighs about 8 grams. The heart, hand, head and feet have developed. It is floating in amniotic fluid within the amniotic sac, and is attached to the placenta (bottom) by its umbilical cord, through which it receives nutrients and where blood gases are exchanged.