The embryo sinks into the thick, spongy lining of the uterus. This will be its home for the next nine months. It is safe here, protected by its mother's body.

The placenta and amnion

When it sinks into the uterus wall, the embryo is only the same size as the egg cell from which it was formed. It has not grown at all. However, it is now made up of many tiny cells, rather than one big cell. These little cells were made as the zygote divided, over and over again. The food stores in the egg cell provided energy for it to do this.
To grow and develop, the embryo now needs more food. A special organ develops that allows it to obtain food and oxygen from its mother's blood. This organ is the placenta. The embryo is attached to the placenta by the umbilical cord.

The little embryo floats in its own private pond, containing amniotic fluid. This fluid is made by a bag called the amnion, which grows around the embryo. The amniotic fluid supports the embryo, and protects it from bumps and knocks.

Foetal development

By the time it is 6 weeks old, the embryo is about $4mm$ long. All its major organs have begun to grow.
At 8 weeks old, the embryo is about $13mm$ long. It is already beginning to move.
At 11 weeks old, all the body organs have developed. The embryo has now become a foetus. It is about $50mm$ long. It is moving quite vigorously now.
From 11 weeks onward, the foetus grows stcadily. Most focuses have finished growing and developing about 38 or 39 weeks after fertilisation happened. The baby is now ready to be born.

Birth

A few days before it is born, the baby usually turns so that it is lying head downwards.
The muscles in the wall of the uterus contract (get shorter). They make the opening of the uterus wider, so that the baby can pass through.
Then the muscles contract in a different way to push the baby out through the opening of the uterus, and through the vagina.