Fertility and Fertilisation

Fertilisation occurs within the fallopian tube. Once the mature egg has been released from the follicle it starts its journey down the fallopian tube. During ejaculation, millions of sperm are released into the vagina. Many will be trapped by the mucous in the vagina; however some continue the journey through the cervix, into the uterus and up into the fallopian tube.

The sperm try to penetrate the egg but only one sperm will penetrate the zona (shell) of the egg. Once the zona has been penetrated, the sperm will lose its tail. A chemical reaction takes place in the egg which prevents other sperm from entering. Once fertilisation has occurred the egg is known as a pronucleate egg.

Pronucleate Egg

The egg continues its journey towards the uterus. During this time it is continually developing and changing, its cells multiplying. As this process of cell division and changes occur the egg has now become an embryo.

The embryo does not implant into the uterine lining until approximately 5-7 days after fertilisation. By this stage the number of cells have multiplied at such a rate that it no longer can be contained within the shell (zona) of the egg, it has become a hollow ball of cells called a blastocyst. The blastocyst then hatches out of the shell (zona) and implants into the lining of the endometrium.