Fertilisation is defined as the event that occurs when a single sperm enters a mature egg, releasing its DNA into the centre of that egg, enabling the two sets of DNA to combine to form a human zygote, a single-celled embryo.
Fertilisation is the first point at which a new human life begins. Some people believe that this is the moment of conception. However, fertilisation does not guarantee pregnancy. The fertilised egg must make a difficult journey from the fallopian tube where fertilisation took place, to the soft lining of the uterus. By the time it reaches here, the fertilised egg will have divided many times, forming a tiny embryo called a blastocyst.
The blastocyst consists of about 120 cells that have all formed from the single zygote produced by fertilisation. It is mature enough to embed itself into the uterine lining and form a placenta to gain nutrients from its mother, and to develop into a fetus and then an unborn baby.
The process of fertilisation is complex. The race for the egg starts with over 500 million sperm that are released into the woman’s cervix during sex. The strongest swimming sperm get ahead, travelling through the cervix, into the uterus and up into one of the fallopian tubes. Fertilisation usually happens here, where the fastest and healthiest of the sperm meet the egg first.
Each sperm has a head and a tail; the head contains an enzyme that is capable of breaking down the tough outer layer of the egg cell membrane, enabling it to breach this defence. This causes an extraordinary change in the egg cell membrane. It then becomes impermeable to all other sperm. No matter how fast they swim and how often they come into contact with the egg, they cannot get in. They all then die within the next two days.
Once the DNA of the egg and sperm fuse to form the zygote at fertilisation, there are many more steps for the fertilised egg to go through. It must make the journey down into the uterus and implant into the thick lining to establish pregnancy. Many pregnancies fail at this point, but if all goes well an early embryo forms, which, nine months later, emerges as a live human baby.