Menstrual cycle is the cyclic change in the reproductive tract of primate females, including human, apes and monkeys. The cyclical changes in the ovary and the uterus during menstrual cycle are induced by changes in the levels of pituitary and ovarian hormones. Menstrual cycle in human female starts at the age of 12-15 years and continues upto the age of 45 years. Menstrual cycle is absent during pregnancy, may be suppressed during lactation and permanently stops at menopause. First menstruation is called menarche and last menstruation is called menopause. In human duration of menstrual cycle is 28 days.
Phases of Menstrual Cycle
The menstrual cycle consists of 3 phases:
1. Follicular phase (Proliferative phase/ preovulatory phase)
2. Luteal phase (Secretory phase/ post ovulatory phase)
3. Menstrual phase (Bleeding phase)
The cycle starts with bleeding phase in its first 3 to 5 days. During this bleeding the part of the layer of endometrium gets shed off. The menstrual flow is a liquid that contain endometrial lining of the uterus, blood. It comes out through vagina. Menstruation only occurs if the released ovum is not fertilised. Lack of menstruation may be indicative of pregnancy. However, it may also be caused due to some other causes like stress, poor health etc. Menstrual fluid contains fibrinolysin, so it doesn't clot.
During this phase, the primary follicles in the ovary grow to become a fully mature Graafian follicle and simultaneously the endometrium of uterus regenerates through proliferation. The secretion of gonadotropins (LH and FSH) increases gradually during the follicular phase, and stimulates follicular development as well as secretion of estrogens by the growing follicles. Both LH and FSH attain a peak level in the middle of cycle (about 14th day). Maximum level of LH during mid cycle is called LH surge. LH surge induces rupture of Graafian follicle and thereby the release of ovum (Ovulation).
During this remaining parts of the Graafian follicle transform as the corpus luteum. The corpus luteum secretes large amounts of progesterone which is essential for maintenance of the endometrium. Such an endometrium is necessary for implantation of the fertilised ovum and other events of pregnancy. During pregnancy all events of the menstrual cycle stop and there is no menstruation. In absence of fertilisation, the corpus luteum degenerates. This causes disintegration of the endometrium leading to menstruation, marking a new cycle.
DRGP Special Points
- Progesterone is known as pregnancy hormone, it is important to maintain the pregnancy.
- The time period between ovulation and next menstrual bleeding is always constant.
- Ovulation occurs at 14th day (mid cycle). In this stage both LH and FSH attain a peak.
- Menstrual fluid contains fibrinolysin, so it doesn't clot.