(a) Since the ovary of female release one egg every month, therefore, the uterus also prepares every month to receive a fertilised egg. The inner lining of the uterus becomes soft and thick with lots of blood capillaries in it. This preparation is necessary as in case the egg is fertilised by the sperm, it helps to keep the egg and nourish it. If however, the egg is not fertilised, then the thick lining of the uterus is not required and the uterus lining breaks down and comes out through the vagina in the form of blood and mucous. This is called menstruation.

(b) Menstruation cycle in females:

(i) When a girl reaches puberty at the age of 10-12 years, the sex hormones released it her blood cause some of the ova in the ovaries to become mature.

(ii) Usually one mature egg is released from the ovary into the oviduct once every 28 days. This is called ovulation.

(iii) Before ovulation, the inner lining of the uterus becomes thick and spongy, and full of blood capillaries, and prepares itself to receive the fertilised egg.

(iv) If the ovum does not get fertilised, then the thick and soft inner lining of the uterus is no longer needed and hence it breaks and the dead ovum comes out from vagina in the form of bleeding called menstruation.

(v) Menstruation usually occurs 14 days after ovulation and usually lasts for 3 to 5 days.

(vi) After menstruation is over, the inner lining of the uterus starts building up again so that it becomes ready to receive the next ovum in case it gets fertilised.

(vii) If the ovum does not get fertilised even now, then the menstruation takes place again and this cycle goes on repeating.