Physiological changes in a fertile woman are necessary for reproduction and occur under the influence of the endocrine gland. This is called the menstrual cycle which occurs in three phases. The menstrual cycle phases are luteal phase, ovulation and follicular phase. They are also known as the secretory phase, proliferative phase and menstruation.
The cycle begins on the 1st day of bleeding (menstruation). As estrogen levels start to increase during the follicular phase, the bleeding becomes slower and finally stops. This is when the uterus thickens. The follicles also start developing because of the interplay of hormones. Only one of the two or occasionally two follicles becomes dominant. As the leutinizing hormones are released, the dominant follicle releases an egg. This stage is called ovulation.
The released egg lives only for 24 hours and sometimes less. Progesterone, another hormone that is released during the cycle, prepares the uterine lining for implantation of the embryo.
In case the implant does not occur, corpus luteum involutes. This means that the estrogen and the progesterone levels drop. So, the uterine lining and the egg are shed, causing bleeding, which is known as menstruation.
Classification of menstrual cycle phases
Bleeding or menses mean that the woman has not become pregnant. Menstruation occurs for about 3 to 5 days but in some cases, it might be 2 days to maximum 7 days. Approximately 35 mm of blood is lost during this stage. There are few women who suffer from iron deficiency because of this bleeding. Hormones like estrogen, follicle stimulating hormone (FSH), progesterone and luteinizing hormone (LH) play a big part in all menstrual cycle phases.
An enzyme called plasmin is also responsible which prevents the blood from clotting. At the beginning or before the menstruation phase begins, the breasts become tender and general mood swings are noticed in most women.
In these menstrual cycle phases, hormones cause uterine linings to proliferate and grow. As the follicular stimulating hormone rises, the follicles, which are present in the ovary, are stimulated to form ovum. Two follicles are present in ovary but only one acquires dominance after the birth and when the woman attains maturity.
Rarely are two follicles dominant at the same time. When follicles mature, they secrete the hormone known as estradiol which initiated the formation of the uterine wall. Estrogen levels, which have also risen, produce cervical mucus.
After the follicular phase and at the beginning of ovulation, estradiol suppresses the luteinizing hormone. Since the egg is matured at this stage, the estradiol levels reach above the threshold and in turn stimulate LH production. As LH surges, the wall of the follicle weakens and ovum is released. Initially, a secondary oocyte is released which matures to ootid and finally becomes the ovum.
This ovum reaches the peritoneal space and is swept by the fallopian fimbria. Nearly after a day, this egg disintegrates if it is not fertilized. If the egg is fertilized in the ampulla, embryogenesis begins and the embryo reaches the endometrium in about 3 days.
This phase is also called the secretory phase. The corpus luteum in these menstrual cycle phases continues to grow for some time after ovulation and finally produces progesterone. It is because of this hormone that the endometrium becomes receptive. FSH and LH which are still present after ovulation transform the dominant follicle into corpus luteum. This then increases the progesterone levels too which triggers estrogen production.
Corpus luteum hormones again suppress LH and FSH. When the progesterone level decreases, menstruation begins.