From zygote to foetus
Fertilisation in the oviduct forms a diploid zygote. As it moves toward the uterus it divides repeatedly by mitosis to form a ball of cells. About a week after fertilisation this ball implants in the endometrium. During the first eight weeks the developing offspring is called an embryo, when the main organs form. From about the ninth week until birth it is called a foetus, when organs grow and mature. A full pregnancy (gestation) lasts about 38–40 weeks.
The placenta and umbilical cord
The placenta is a disc-shaped organ that grows in the uterus wall. It brings the mother's blood and the foetus's blood very close together without mixing them. Across this thin barrier, useful substances such as oxygen, glucose, amino acids and antibodies diffuse from mother to foetus, while wastes such as carbon dioxide and urea pass from foetus to mother. The umbilical cord links the foetus to the placenta and contains blood vessels.
Protection of the foetus
The foetus floats in amniotic fluid held in the amniotic sac. This fluid cushions the foetus against knocks, allows free movement so the limbs develop properly, and helps keep its temperature steady. At birth the sac breaks, the muscular uterus contracts and pushes the baby out through the cervix and vagina, and the umbilical cord is cut. The placenta, now called the afterbirth, is delivered shortly afterwards.
Example
Alcohol, nicotine and some viruses can cross the placenta from the mother's blood, which is why pregnant women are advised not to smoke or drink alcohol.
Key idea
Embryo (weeks 1–8): organs form. Foetus (week 9 to birth): organs grow. The placenta exchanges materials by diffusion but keeps the two blood supplies separate.