Menstrual Cycle

Female Reproductive System

Hormone Levels

Hormone imbalance is best understood by knowing how a normal menstrual cycle works. A menstrual cycle is the result of a hormonal dance between the pituitary gland in the brain and the ovaries. Every month the female sex hormones prepare the body to support a pregnancy, and without fertilization there is menstruation (a period).

Getting To Know Your Body

Luteinizing Hormone

The follicle produces more estrogen to prepare the uterus for pregnancy. At ovulation, usually around Day 12 – 14, increased estrogen levels trigger a sharp rise in Luteinizing Hormone (LH) from the pituitary gland, causing release of the egg from the follicle. Also released from the pituitary gland in the brain at ovulation, and causes the rupture of the mature ovarian follicle, releasing the egg.

Follicle Stimulating Hormone

On Day 1 of the menstrual cycle, estrogen and progesterone levels are low. Low levels of estrogen and progesterone signal the pituitary gland to produce Follicle Stimulating Hormone (FSH). FSH begins the process of maturing a follicle (fluid-filled sac in the ovary containing an egg). Released from the pituitary gland in the brain, and stimulates the ovarian follicles (fluid-filled sacs on the ovary containing an egg or ovum) to mature.

Progesterone

Another of the female sex hormones. It works in the body to balance the effects of estrogen and is often referred to as the relaxing hormone. Progesterone is produced after ovulation by the corpus luteum (sack that the egg comes from) and dominates the second half of the cycle (luteal phase). Progesterone¹s main job is to control the build up of the uterine lining and help mature and maintain the uterine lining if there is a pregnancy. If there is no pregnancy, our progesterone levels fall and the lining of the uterus is shed, beginning the menstrual cycle.

Estrogen

One of the female sex hormones and often referred to as the ³growing hormone² because of its role in the body. Estrogen is responsible for growing and maturing the uterine lining (lining that is shed during menstruation) and also matures the egg prior to ovulation. Estrogen is produced mostly by the ovaries but also in smaller amounts by the adrenal glands and in fat tissue. It is most abundant in the first half of the menstrual cycle (follicular phase).

Endometrium Lining

The endometrium changes throughout the menstrual cycle in response 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 (ovulation). 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 menstrual cycle begins.

Follicular Development Based on Hormones

The production and release of follicle stimulating hormone is regulated by the levels of a number of circulating hormones released by the ovaries and testes. This system is called the hypothalamic-pituitary-gonadal axis.


In women, when hormone levels fall towards the end of the menstrual cycle, this is sensed by nerve cells in the hypothalamus. These cells produce more gonadotrophin-releasing hormone which in turn stimulates the pituitary gland to produce more follicle stimulating hormone and luteinising hormone and release these into the bloodstream. The rise in follicle stimulating hormone stimulates the growth of the follicle in the ovary. With this growth, the cells of the follicles produce increasing amount of oestradiol and inhibin. In turn, the production of these hormones is sensed by the hypothalamus (or pituitary gland in the case of inhibin) and less gonadotrophin-releasing hormone and follicle stimulating hormone will be released. However, as the follicle matures, there is a surge in oestradiol production which switches the feedback mechanisms in the brain from negative to positive feedback, causing more gonadotrophin-releasing hormone, follicle stimulating hormone and luteinising hormone to be released. This peak in hormone levels eventually leads to ovulation.

Pregnancy & Hormones

Estrogen and progesterone are the chief pregnancy hormones. A woman will produce more estrogen during one pregnancy than throughout her entire life when not pregnant. The increase in estrogen during pregnancy enables the uterus and placenta to improve vascularization, transfer nutrients, and support the developing baby. In addition, estrogen is thought to play an important role in helping the fetus develop and mature. Estrogen levels increase steadily during pregnancy and reach their peak in the third trimester. The rapid increase in estrogen levels during the first trimester may cause some of the nausea associated with pregnancy and, during the second trimester, plays a major role in the milk duct development that enlarges the breast.

Changes in Lining

The lining of the uterus has thickened to prepare for the fertilized egg. If no conception occurs, the uterine lining as well as blood will be shed. The shedding of an unfertilized egg and the uterine wall is the time of menstration