Hello, thanks for posting your query.
I am impressed with your curiosity. Puberty is the transitional phase linking the childhood and adulthood which involves not just menstrual period but also physical, biological and psychosexual changes in a girl.
The changes during the puberty involve the changes in the body shape, increase in the breast size, growth spurt and finally the menstrual period. Period is a process which is normal in every woman once she attains the puberty. It is a regular monthly or a 28 day cycle ongoing phenomenon and continues until she reaches menopause.
This process is influenced by the interaction of various hormones which include estrogen, progesterone, growth hormone, thyroid hormone, follicle stimulating hormone etc. Under their influence, the ovary responds and starts ovulating. The ovary ovulates one egg per month. This ovum is taken up the tube where the ovum stays for a while and waits for the sperm to unite with.
During this period which takes around 2 weeks the uterus prepares itself (like bedding for the new embryo) for implantation of the egg, and when the egg does not unite with the sperm, the uterus just sheds its bedding layers. These layers are richly supplied with blood and thus there is shedding of blood. The menstrual blood consist not only blood, but also has the dead debris. The uterus sheds bloody tears because there is no embryo formation.
But once the woman is sexually active, the embryo is formed and the bedding is ready in the uterus for the implantation. Then the uterus does not shed any more blood- leading to missing period- which means the women is pregnant.
Hope this helps.