So basically menstruation is a female reproductive cycle which occur in all female primates like monkeys human apes and it is known as menstrual cycle.I will keep it very simple to make you understand better. The menstruation generally starts at the age of 10-12 years which is starting of adolescence age.It generally last for 3-5 days and is repeated at every 28-29 days on an average and there is bleeding through vagina in it. No need to worry it is very natural and shows that you are reproductively healthy. If it not happens there might be any problem and you should see a physician. First i will tell you what is it and how it happens then i will share my experience of first period. why does menstruation happens? so as we know that it is repeated at every 28-29 days it means every girls has her periods(menstruation is also know as periods) every month her body releases an egg which needs to get fertilize by sperm(from man)if this does not happen the egg will no