Menstrual Cycle 101

To understand just about anything related to women's health, you need to understand the menstrual cycle. From puberty to birth control (contraception) , infertility and menopause, these processes are at the center of it. 

Consider the example of contraception, especially hormonal contraception. You must be able to think of 3 organ systems at once:

1. The brain

2. The ovaries and their follicles

3. The uterus and its lining, called the endometrium

 I suggest referring to the menstrual cycle infographic during this recording. 

One can best understand the menstrual cycle by tracking it on a 28 day calendar. On this calendar you have to superimpose the activity of the uterine lining, or endometrium, the activity of several hormones, and the activity of the ovary. 

For an audio reading of this information, click below. Sometimes it is easier to understand if you look at the graph while hearing the explanation.

A print version is below.

The follicular phase

So let’s begin by defining day 1 as the first day of the last menstrual cycle. That is the first day that one sees any bleeding at all. On this day the endometrial lining begins to evacuate. It starts thick and ends thin over the course of about 5 days. Already preparations are being made for the new cycle. Remember the whole purpose of the menstrual cycle is to allow a woman to become pregnant. In preparation for this, follicle stimulating hormone (FSH)  is being secreted from the anterior pituitary of the brain, and as the name suggests, it stimulates follicles. Follicles are simply the little sacs within the ovary which hold the ripening eggs. As the follicles ripen in preparation for ovulation, estrogen is produced by the ovary and stimulates the endometrium to re-thicken in preparation to nourish an incoming pregnancy. To recap, during the follicular phase, FSH stimulates the eggs to ripen, and estrogen thickens the endometrium. 

Ovulation 

Shortly before day 14, luteinizing hormone (LH) from the anterior pituitary pulses into the circulation, normally stimulating ovulation. Ovulation occurs, which means the egg is released from its follicle. The egg is either fertilized and implants or just washes away.  

The Luteal Phase 

Let’s assume no fertilization takes place. The tissue that the egg leaves behind, the empty follicle, becomes a progesterone secreting factory so active that it takes on the yellow color of progesterone. We call it the corpus luteum, which in Latin means yellow body. Similarly, we call this whole phase from ovulation to the next new menstruation, the Luteal Phase. During the luteal phase progesterone travels through the circulation and acts on the endometrium to compact it. After 14 days, it is ready to slough off in the form of the next menstrual period and a new day 1 is assigned. 

Under normal circumstances the follicular phase is about 14 days. A variation of plus or minus 2 or 3 days is not uncommon, and is considered normal. However the length of the luteal phase is fixed. Once an ovulation takes place, a biological clock is set ticking, and exactly 14 days will elapse until the next period. The menstrual interval is the number of days in the follicular phase plus the number of days in the luteal phase. Therefore the average menstrual interval is 28 days. If menstrual interval length varies, it is accounted for by variation in the length of the follicular phase. Basically, when interval length varies, it is a statement about how long it takes the patient to ovulate. One can use this information prospectively to try to estimate when ovulation occurs. Indeed over-the-counter ovulation test strips check for, you guessed it, the LH surge. To recap, during the luteal phase, LH is secreted, ovulation occurs, the corpus luteum secretes progesterone which compacts the endometrium in preparation for the next period. 

How do hormonal methods of contraception affect the menstrual cycle? 

 Before I can answer this question, I must explain the concept of a feedback loop. As I mentioned before, our menstrual cycle begins in the brain when FSH is released into the circulation. FSH of course stimulates the follicle to ripen and the ovary to produce estrogen. The new piece of the puzzle is this: While the estrogen enters the circulation and thickens the endometrium, it also tells the pituitary to slow down on the FSH production. This is an example of a negative feedback loop. There are many of these hormonal and chemical feedback loops in the body and they are fascinating. Their purpose is to keep us in a state of dynamic equilibrium. With this background information, we can be precise about how the very popular combination oral contraceptive pill works: 

When we take hormone containing birth control, we give negative feedback to the pituitary, shutting down FSH and LH, thereby inhibiting ovulation. No ovulation, no conception, no pregnancy. For more information, see Hormonal Birth Control. 

Understanding the menstrual cycle is useful for understanding both normal and abnormal menstruation such as abnormal uterine bleeding. It is also useful for understanding metabolism, ( see metabolic syndrome) menopause, infertility and health in general.