Menstruation, or the menstrual period, is the monthly process by which a woman’s body rids itself of blood and tissue that builds up in her uterus in preparation for pregnancy. Most women experience their first period between the ages of ten and thirteen years old. Like many processes in the body, fertility and reproduction are controlled by the body’s hormonal levels. As children undergo puberty, the changes they experience are caused by hormonal fluctuations. In girls, these hormonal fluctuations are comprised primarily by increases in the amount of estrogen, progesterone, follicle stimulating hormone (FSH) and luteinizeing hormone (LH) released in the body. Each of these hormones influences a component of reproductive activity in the body, and they work in concert to create a cycle, usually between 28 and 35 days in length, during which the lining of the uterus sloughs off and grows again in preparation for pregnancy.
The cycle begins with the first day of bleeding, when FSH is released by the pituitary gland and begins stimulating the "ripening" of eggs in the ovaries. Estrogen is then released from the ovaries, making the cervix (the opening to the uterus at the top of the vagina) and cervical mucous more amenable to penetration by sperm.
When estrogen levels in the body are high enough, the pituitary gland releases LH, which causes the "ripest" egg to be released from the ovary and travel to the fallopian tube (the tube that runs between the ovaries and the uterus) to await fertilization. After the egg is released, the follicle that burst to release it, now called the corpus luteum, continues to secrete a lesser amount of estrogen, which acts to return the cervix to its infertile state.
The corpus luteum also releases increased amounts of progesterone, which causes the lining of the uterus, or endometrium, to thicken in preparation for the implantation of the fertilized egg in the uterine wall, as well as stopping the ovaries from producing eggs and returning the cervical mucous to normal.
