Why does menstruation occur? Describe the menstrual cycle in human females (or women).

Menstruation occurs due to the rupture or removal of the inner, thick and soft lining of the uterus when fertilisation does not take place.
The menstrual cycle of a female is as follows:

• After a female attains puberty, every month one of the ova of the ovary matures and gets released once in every 28 days by the process of ovulation.
• The lining of the uterus becomes thick and spongy before ovulation in order to receive the fertilised egg. If the egg is not fertilised, this lining of the uterus breaks and comes out of the vagina in the form of blood known as menstrual flow.
• This occurs on the 14^thday of ovulation and lasts for 3-5 days.
• After this flow stops, the uterus starts preparing itself again for the fertilised egg. If it does not receive the fertilised egg, the same set of events are repeated.