Fertilisation

During copulation (sexual intercourse), the penis is inserted into the cervix and ejaculates sperm cells which swim from the cervix into the fallopian tubes. If an ovum is present in the fallopian tubes, one sperm cell may penetrate through the jelly layer and fertilise the ovum, resulting in a diploid zygote, due to the fusion of the sperm cell and ovum cell, known as fertilisation.

The zygote will go through mitosis as it moves down the fallopian tube, towards the uterus. Mitosis will occur until the morula ( a solid ball of undifferentiated cells) is formed. The morula will then form a blastocyst, a hollow fluid-filled ball. It takes approximately 5 days to form the blastocyst.

Implantation then occurs, where the blastocyst travels from the fallopian tubes to the uterus. The outer cells of the blastocyst will secrete enzymes to break down a small portion of the thickened uterine wall, making it softer. The blastocyst will then sink itself into this softer area, and the two outer layers will form into two extra-embryonic layers (amnion and chorion).

The chorion (outer layer of extra-embryonic layers) will extend into outgrowths, called chorionic villi, into the endometrium, and form part of the placenta, which secretes progesterone. Now the blastocyst is called an embryo.

After 12 weeks, the embryo is known as a foetus.


The amnion (inner layer of extra-embryonic layers) will become filled with amniotic fluid to form the amniotic sac. The amniotic fluid :

  • protects the foetus from mechanical injury by absorbing mechanical shock

  • prevents dehydration

  • thermoregulation of the foetus by only allowing for a minimal temperature range

  • allows for free-movement for the foetus as it develops and grows allowing for muscle growth


The allantois is an extra embryonic layer responsible for the excretion of waste within an egg.


The umbilical cord connects the foetus to the placenta and contains umbilical blood vessels, known as an umbilical vein which carries oxygenated blood, nutrients, water and other substances from the placenta to the foetus, and two umbilical artieries, which carry deoxygenated blood and waste products from the foetus to the placenta.

The placenta is a temporary organ located at the original site of implantation of the blastocyst. This acts as the point of attachment of the foetus to the mother. The placenta also allows for substances to be transferred by diffusion between the mother and the foetus without any blood being in direct contact, meaning that no blood between the mother and foetus ever mixes. This means that diffusion of nutrients (mother to foetus), gas exchange (O2 to foetus, CO2 to mother) , and waste products (foetus to mother) is able to occur. The placenta also ensures that after 12 weeks progesterone is secreted to maintain the pregnancy by ensuring that the endometrium stays thick and stimulate the mammary glands for milk production. Also produces antibodies that pass to the foetus, and it acts as a microfilter to pathogens except for harmful substances (alcohol and nicotine) and some viruses (HIV & German measles).


Alcohol can lead to Foetal Alcohol Syndrome, which

Also produces oestrogen to help muscles in the uterus wall to contract during the birth process.


Gestation - time in which the embryo develops inside the uterus; lasts for about 40 weeks / 280 days