Normally, the menstrual cycle starts at age thirteen. Some girls begin to menstruate as early as eleven years old or later at fifteen. On the average, it takes about 28 days to complete one menstrual cycle although it may range from 22 to 36 days. Every month, each of the ovaries releases an egg. This process is called ovulation.
The hypothalamus controls the hormones responsible for the menstrual cycle. These hormones are as follows:
1. The Follicle-Stimulating Hormone (FSH)
The follicle-stimulating hormone is produced by the pituitary gland. It enlarges the ovaries and the ovarian follicles. It causes the ovaries to produce estrogen.
2. Luteinizing Hormone (LH)
The luteinizing hormone enables the ovary to release an egg or ovum. This process occurs fourteen days after the menstruation begins in a woman with a 28-day cycle.
3. Progesterone and Estrogen
The cells lining the empty follicle rapidly increase in number. They become changed to lutein cells forming the corpus luteum. They secrete progesterone and estrogen. These hormones cause the endometrium, the lining of the uterus, the double its thickness in blood.
If no fertilization occurs, the blood in the endometrium breaks apart and comes out of the body in the form of menstruation. It may last from three to seven days. Then, the cycle begins again.
However, if the egg cell is fertilized by the sperm cell, conception occurs and the girl becomes pregnant.