The Menstrual Cycle

The Menstrual Cycle

Very few women truly understand how complex their bodies are. Amongst the myriad of signals, hormones, and sequential steps that need to happen in conditions that are just right, lay some key aspects to understanding your menstrual cycle. The basic function of a cycle is to prime the body for a pregnancy. Ideally this happens through a cyclical and predictable pattern of ovulation followed by menstrual bleeding; the cycle is theoretically repeated unless interrupted by pregnancy. In reality many women find that they are plagued by spotting, cramping, and outright bleeding at varying times throughout the month. The variety of things that can cause an irregular period is diverse and can be frustrating to both the patient and their doctor. Being overweight, underweight, hyperthyroid, hypothyroid, medications, drug use, and anatomic and chromosomal abnormalities can all impact a cycle.

The average length of an adult menstrual cycle as measured from the beginning of a menstrual bleed to the beginning of the next menstrual bleed is approximately 28 days with a normal cycle defined as 23 to 35 days. The onset of menstrual bleeding is termed the first day of the menstrual cycle and is the follicular phase. This is when the body secretes follicle stimulating hormone (FSH) which stimulates ovarian follicle development for the first 10-13 days of the cycle. The ovarian follicle contains the egg that needs to slightly mature in order for ovulation and possible fertilization. When a woman is born she has approximately 1 to 2 million of these immature eggs. By the time she begins menstruating there are approximately 300,000 to 500,000 immature eggs remaining. Throughout her life these eggs will be lost through ovulation or natural degeneration processes and once the reserve of these eggs is diminished menopause will follow.

While all this is going on in the ovary, the rest of the menstrual mechanisms are not standing idly by. There is a gradual build up of leutinizing hormone (LH) culminating in the “LH surge” on days 11-13. Once the surge begins ovulation will follow 34 to 36 hours later with some women being able to feel a twinge of pain at time of ovulation. Couples who are trying to conceive are looking for this “surge” in order to time intercourse and this is what the ovulation predictor kits are designed to detect.

If fertilization occurs then the process is halted and resumed after the pregnancy is completed. If there is no implantation of a pregnancy then hormonal changes will lead to starvation of the innermost layers of the uterus causing breakdown of the endometrium and menstruation to begin. It is important to note that not all vaginal bleeding is menstrual bleeding. Now while this sounds counterintuitive, the most common cause of abnormal bleeding is annovulatory bleeding. This happens when there is an irregularity causing ovulation to misfire and not occur. When this happens the normal order is disrupted and the lining of the uterus does not come off. Eventually the lining will be too thick for its blood supply to nourish and will cause parts of the endometrium to come off at irregular times leading to irregular bleeding. Although this is the most common cause it is important to follow up with your doctor because cancer, infections, etc can also be associated with vaginal bleeding.

This is a very simplified outline of the immensely delicate and intricate ballet that must be executed monthly in a woman’s body. The hormones above interact with each other in very complex fashions along with a variety of other hormones and biochemical processes enabling a woman to enact her reproductive rights. See your doctor if you have any questions or suffer from irregular or painful menstrual cycles.