Once a month your body prepares itself for pregnancy. If you aren't pregnant, you have a period. Blood and mucus from your uterus are discharged instead of being used for a pregnancy.
From ovulation to fertilisation
Every month an ovary releases a mature egg into the fallopian tube. We call this ovulation, and it happens around 2 weeks after your period. If you have had sex without using contraceptives, and a sperm has found its way into a fallopian tube, fertilisation may take place. You become pregnant. The fertilised egg travels along the fallopian tube to the uterus.
Bed of mucous membrane
Once a month your hormones simultaneously ensure that the wall of your uterus develops a thick layer of mucus and blood. It is actually like a bed of mucous membrane. If you become pregnant, the egg that has been fertilised by the sperm has a soft landing on that bed and snuggles down in it.
No fertilisation: a period
If no egg is fertilised and you’re not pregnant, that bed of mucous membrane has no function anymore. That is why your body repels the inner wall of the uterus: blood and mucus come out of the vagina. This blood and mucous have actually come from your uterus.
Your menstrual cycle starts with your period. That is day 1.
Ovulation: approximately 2 weeks later an ovary releases an egg into the fallopian tube. This egg then travels to the uterus. You can get pregnant around this time.
If the egg isn’t fertilised by a sperm, it dies. Approximately 2 weeks after ovulation, the egg and the mucous membrane pass out of your uterus: you have a period again.