What is menstruation ? Explain.

Before a zygote is received in the uterus, the uterus wall becomes thick with a lining of soft and spongy tissue full of blood vessels. In case fertilisation does not take place, the ovum along with the lining and blood vessels along the uterus wall shed off. This results in bleeding in women which continues for 2 to 3 days. This is called menstruation.