Fertilisation and pregnancy

Fertilisation is the fusion of a haploid sperm nucleus and a haploid egg nucleus to form a diploid (normal chromosome number) zygote.

A zygote is the first cell of a new individual. Fertilisation occurs in the oviduct.

After fertilisation:

  • The zygote travels down the oviduct dividing by mitosis to form a ball of cells (embryo).
  • When the embryo reaches the uterus, implantation occurs. This is when the embryo attaches to the thick uterus lining to receive nourishment.
  • The placenta, umbilical cord, amnion and amniotic fluid form.
  • The embryo differentiates to produce a variety of tissues and organs.
  • The embryo is referred to as a foetus when it begins to look more like a baby.

Placenta and umbilical cord

The placenta allows substances to diffuse from the mother’s blood to the foetus (e.g. oxygen and glucose).

Substances can also diffuse from the foetus to the mother’s blood (e.g. carbon dioxide and urea).

The placenta is adapted for diffusion by having:

  • A large surface area between it and the uterus wall.
  • Villi (finger like projections that extend into the uterus wall), which further increase the surface area of the placenta.
  • A rich supply of maternal blood vessels.


The umbilical cord attaches the placenta to the foetus. It contains the umbilical artery and the umbilical vein.

The umbilical artery carries urea and carbon dioxide from the foetus to the mother’s blood.

The umbilical vein carries oxygen and nutrients from the mother’s blood to the foetus.

Maternal blood and foetal blood systems are close together but not joined. They are separated by thin membranes that allow diffusion to occur.

Amnion and amniotic fluid

The amnion is a membrane that forms around the embryo.

Amniotic fluid surrounds the embryo inside the amnion, cushioning and protecting the developing embryo.