Illustration of a human embryo in the uterus (womb). The developing baby is an embryo from just after conception to around 8 weeks (centre). It is then known as a foetus from around 8 weeks until birth. The embryo is floating in amniotic fluid within the amniotic sac. It is attached to the placenta (outlined in pink) by its umbilical cord, through which it receives nutrients and where blood gases are exchanged.