Fertilization

It is the fusion of the nuclei in a sperm and ann egg to form a fetus.
During this process, the chromosomes in the sperm and the egg fuse together to form a full set of 23 pairs of chromosomes.
Millions of sperm released from the penis are deposited in the vagina.

The sperms swim from the vagina up the uterus and into the fallopian tubes, many sperms actually die before they reach the tubes.
If a sperm meets an egg in a fallopian tube, fertilization takes place, only one sperm will fertilize the egg.
The fertilized egg begins to divide to form a small ball of cells.
The small ball of cells, now called the embryo, moves down the fallopian tube and enters the uterus.
The embryo secretes a hormone which prevents the corpus luteum from breaking down until a structure called the placenta is formed. the corpus luteum continues to secrete progesterone and oestogen until the placenta is able to take over the production of progesterone and oestogen.
The embryo embeds itself in the uterine lining and develops.

In the event that fertilization does not take place, the egg will break down.
Eventually, LH production is inhibited by high concentration of progesterone in the blood.
The drop in LH level in the blood causes the corpus luteum to break down too.
When the corpus luteum breaks down, progesterone is not secreted.
Without progesterone, the thickened uterine lining cannot be maintained, so it breaks down.
The lining flows from the uterus together with some blood through the vagina, this marks the beginning of the previous menstruation.
FSH is again produced by the pituitary gland and the whole cycle is repeated.