In a human female, the fertility period extends from the age of puberty, i.e. about 12-13 years up to menopause, i.e. 45-50 years. The stage of puberty is marked by the appearance of secondary sexual characteristics.
Thus, puberty is a period in which reproductive system matures and becomes capable for reproducing. At the time of menopause, ovulation and menstruation stop and the reproductive organs decrease in size.
During each menstrual cycle, an ovum is matured and released once every 28 days. The period of menstrual cycle is counted from the day of onset of menstrual flow to the next onset after about 28 days. There are four phases of menstrual cycle as given below.
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1. Menstrual Phase:
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The menstrual cycle starts with the menstrual flow, during which the cellular lining of the uterus, with blood flow, is shed off. This process continues for 3-4 days. During menstrual phase the ovary starts to form a new egg in follicle.
2. Follicular Phase:
From the 5th up to the 13th day of the onset of menstrual cycle, growth and maturation of the graafian follicle takes place. Graafian follicle is the final stage in the maturation of an ovum inside the ovary. It consists of an ovum and a mass of cells surrounding it.
The Graafian follicle produces a hormone, oestrogen. This hormone stimulates the uterus to prepare itself to receive the ovum. The cells lining the uterus grow rapidly and develop a dense network of blood vessels.
3. Ovulatory Phase:
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In this phase ovulation takes place. The Graafian follicle ruptures to release the ovum. The cells of the ruptured follicle form the corpus luteum which secretes the hormone, progesterone. The release of the ovum from the ovary is called ovulation. The ovum reaches the uterus via the fallopian tube on the 13th or 14th day and remains there up to the 16th day (for 48-72 hours).
4. Luteal Phase:
If the ovum does not get fertilized by any sperm during ovulatory period then it starts degenerating. At the end of the 28th day this ovum is rejected along with the uterine lining. This marks the start of a slow disintegration of the thickened lining of the uterus and the next menstrual cycle. The empty graafian follicle in the ovary turns into corpus luteum.