CBSE Class 12-science Answered

Before puberty, a girl's reproductive system has not developed and she does not carry out ovulation. During puberty, the reproductive system as well as her secondary sexual characteristics develop, which includes menstruation.

Menstruation occurs only if the ova released during ovulation has not been fertilised. Then this ova is removed from the body through menstruation. Pregnancy results when the ovum has been fertilised by the sperm. Hence it does not have to be removed. To prevent menstruation , levels of progesterone and oestrogens rise continually throughout pregnancy, suppressing the hypothalamic axis and subsequently the menstrual cycle.

Menopause marks the permanent cessation of reproductive fertility some time before the end of the natural lifespan. The female is no longer fertile after menopause and does not show ovulation. Hence menstruation also stops. There is great decrease in the levels of circulating estrogen after menopause.

A male who has reached puberty will produce millions of sperm cells every day.It takes X amount of sperm cells to successfully fertilize an egg. A good number of sperms fail to reach the destination. So men produce many sperm to have multiple tries at mating. In each mating session, most sperm will never make it to the ovum. Having more sperm increases the odds of fertilization.