After fertilisation, the newly-formed zygote divides repeatedly to form a ball of cells called an embryo. This becomes implanted in the wall of the uterus.
After eight weeks of development, the embryo is called a fetus (also spelt ‘foetus’ but pronounced the same). The amniotic sac produces amniotic fluid, which surrounds and protects the developing embryo.
An ultrasound image of a developing fetus
A placenta, connected by an umbilical cord, develops from the embryo. The placenta anchors the embryo in the uterus. It also allows:
nutrients and oxygen to move from the mother to the embryo
waste materials and carbon dioxide to move from the embryo to the mother
There is no physical connection between the circulatory systems of the embryo and its mother, so their blood doesn’t mix. These materials pass from one to the other by diffusion.