Menstrual Cycle

Anatomy & Physiology

How hormone levels fluctuate throughout the cycle.

During perimenopause hormone levels fluctuate as a result of fewer ovulations, so less progesterone is produced in the second half of the menstrual cycle. Periods can be erratic, skipped or have heavy bleeding /clots. Symptoms result from the change in ratio of estrogen to progesterone ­ so the imbalance creates the symptoms.

How endometrial lining changes due to hormones.

During the first part of the cycle, the hormone estrogen is made by the ovaries. Estrogen causes the lining to grow and thicken to prepare the uterus for pregnancy. In the middle of the cycle, an egg is released from one of the ovaries. Following ovulation, levels of another hormone called progesterone begin to increase. Progesterone prepares the endometrium to receive and nourish a fertilized egg. If pregnancy does not occur, estrogen and progesterone levels decrease. The decrease in progesterone triggers menstruation, or shedding of the lining. Once the lining is completely shed, a new cycle begins.

How follicular development occurs based on hormones.

During each menstrual cycle there is a rise in follicle stimulating hormone secretion in the first half of the cycle that stimulates follicular growth in the ovary. After ovulation the ruptured follicle forms a corpus luteum that produces high levels of progesterone. This inhibits the release of follicle stimulating hormone. Towards the end of the cycle the corpus luteum breaks down, progesterone production declines and the next menstrual cycle begins when follicle stimulating hormone starts to rise again.

How hormone levels change due to pregnancy.

The hormone and physiologic changes during pregnancy are unique in the life of women.

Pregnant women experience sudden and dramatic increases in estrogen and progesterone, as well as changes in the amount and function of a number of other hormones. These changes can not only affect mood, they can also create the “glow” of pregnancy, significantly aid in the development of the fetus, and alter the physical impact of exercise and physical activity on the body.

How endometrial lining changes due to pregnancy.

During pregnancy, the glands and blood vessels in the endometrium further increase in size and number. Vascular spaces fuse and become interconnected, forming the placenta, which supplies oxygen and nutrition to the embryo and fetus.

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