Once the spermatozoa have been deposited into the vagina by ejaculation, millions of spermatozoa begin an epic race to reach and fertilise the ovum. This race really is a challenge, considering that of the thousands of millions of sperm which start out, only one will manage to fuse with the egg.
From the vagina the spermatozoa travel to the Fallopian tubes, and it is here that those spermatozoa which make it will encounter the ovum, and where fertilisation will take place. This journey is possible thanks to the fact that sperm are able to move because of their tail, and also as a result of contractions of the walls of the vagina and uterus, which facilitate their movement.
When they meet the ovum, only some two hundred of the millions of spermatozoa which were deposited during ejaculation will have made it. These surround the egg and begin to secrete a protein enzyme (hyaluronidase), the function of which is to break through the external membrane of the ovum. Only one of these sperm will be able to entirely pierce the membrane and fully enter the ovum. When it achieves this, it leaves its tail behind and moves towards the nucleus of the ovum, and at the same time it activates substances which will prevent other sperm from entering into the egg.
The ovum remains capable of being fertilised for approximately 24 hours after ovulation takes place. The spermatozoa stay viable in the cervical mucus for approximately 48 - 72 hours.
Fertilisation occurs when the nuclei fuse together, that is the nucleus from the head of the sperm which has penetrated the ovum, and the nucleus of the ovum itself. As such, 46 chromosomes succeed in being brought together (each sexual cell provides 23), which constitutes the chromosome set of the new individual. Although half of the genetic material corresponds to each of the parents, the union leads to the creation of a truly unique being, which at this stage is called a zygote.
The fertilised egg will develop in the Fallopian tubes during the first three days before it moves down to the uterus. On the fifth day it becomes what is known as a blastocyst, an embryo of between 70 – 100 cells which is ready to be implanted. On the 6th or 7th day after fertilisation the blastocyst breaks free from its protective membrane (also known as the zona pellucida), which it needs to do to achieve the necessary conditions for adhering to the surface of the endometrium, and at the same time it begins to secrete human chorionic gonadotropin (hCG), a hormone which tells the corpus luteum to continue producing progesterone.