[Thomas the Rhymer]

The Female Menstrual Cycle - A brief introduction

Let's start with the TL;DR version:
In the Forum Lounge thread, there was some discussion as to when exactly a woman is fertile in her menstrual cycle. This thread is a response to that question.

The answer is: from the date of ovulation (egg release) to the time of her next period. Generally a period of fourteen days.

Note that the date of her last period is irrelevant - what matters is the time of the next period.

Now, exactly deducing this timing is exceptionally difficult for the following reasons: most women do not have the textbook 28 day cycle; most women have slight variations in their cycle time anyway; events in a woman's life can disturb the normal cycle; sometimes the cycle just messes up for no reason. Like economics forecast, trying to figure out the exact date of the next period is often a vague and imprecise exercise.

The time of ovulation is also somewhat unpredictable; but in general, it is fourteen days before the next period. Give or take a day, this is a reasonably hard and fast rule.

For a woman in a textbook 28 day cycle (note that the textbook cycle, in my clinical experience, does not actually exist; it primarily exists as an educational model, not a real world reflection) : she will be fertile for the 14 days before her next period. Assuming she has a 28 day cycle, that will mean she is infertile for the first half/fourteen days of her cycle (which starts on the first day of her last period). She will then be fertile for the next fourteen days/second half of her cycle (which lasts up the first day of her next period.)

Random examples:

For a woman with a 21 day cycle: she will 7 days infertile, then 14 days fertile.

For a woman with 34 day cycle: she will be 21 days infertile, then 14 days fertile.

So fertility is basically the 14 days before a period. After a period, whether she is fertile or not depends purely on the timing of the next period.

Timing when the next period and/or ovulation will occur is referred to as the rhythm method. Because so many factors can cause the period to come earlier or later than expected, the rhythm method is notoriously unreliable to the point where pretty much every medical student is taught this joke to bring home the point:
Q: What do you call patients that decide to use the rhythm method of contraception?
A: Parents.

This is further complicated by the fact that the life span of sperm within the female reproductive tract is variable, but as a rule of thumb, sperm can survive for up to seven days in the female reproductive system. Meaning that even if the woman is not fertile then and there, your sperm may live long enough to catch the egg once it is released. I.e. you may only end up impregnating a woman seven days after intercourse.

In other words, gents, the only time a girl will be well and truly infertile will be during her period, as any egg and sperm will be washed out with the blood. At any other time of her cycle, surprises can and do happen in the real world.

Okay, that was the TL;DR version. Now let's get into the nitty gritty. I will avoid using medical jargon and try to stick to plain English as much as possible.

First up: The Anatomy (very much in brief)

Here's a pic from wikipedia:


The vagina ends at the cervix. The cervix functions as a gatekeeper into the womb, and can make life easy or difficult for sperm depending on the stage of the menstrual cycle.

The uterus is where babies grow, and is also responsible for menstrual bleeding (hence why menstrual bleeding is also referred to as uterine bleeding).

The ovaries are responsible for hormone production as well as egg release. Much of the uterine cycle is in response to ovarian hormones.

And now, The Ovarian Cycle

So there are multiple cycles in a woman's body: the uterine/menstrual cycle, the breast cycle, the cervical cycle, the skin cycle, etc. But they are all secondary to the brain-ovarian cycle, which is the cycle that rules them all.

So briefly, in as plain English as possible, this is the cycle:

1) The brain detects that there is a low level of oestrogen and progesterone in the blood. The brain freaks out and tells the pituitary gland to activate.

2) The pituitary gland decides to send a message to the ovary, telling it to make more oestrogen. Oestrogen is made in follicles in the ovary. So the pituitary gland releases Follicle Stimulating Hormone, or FSH, into the bloodstream.

3) FSH then activates the development of a bunch of follicles, or stimulates follicles that are already matured.

At this point, you are asking, "Dude, what the heck is a follicle?"

Good question. To keep the discussion simplified, the follicle is basically: A)an egg surrounded by B)a shell of tissue, referred to as the granulosa.

Think of an egg you buy in the supermarket, but instead of a hard shell, the human egg shell is made of body tissue.

Now just as a bird's egg shell is specialised to be a hard protective casing, so too does the human egg shell have a special function - it secretes Oestrogen! The vast majority of a woman's oestrogen comes from these egg shells.

(Now it should make sense to you why, once all the eggs are used up, a woman will enter menopause - because without any eggs, she won't have any egg shells left, and therefore no capacity to make oestrogen).

So that's a follicle. Most eggs in the young adult woman will be dormant with a very tiny shell of tissue, but clumps of them will develop with every cycle under the influence of Follicle Stimulating Hormone. (Eventually, one follicle will become the dominant follicle, will swell up, and burst, releasing the egg. But more on that a little later)

4) As the follicles develop, the granulosa shells of the eggs grow, become more complex, and start secreting oestrogen.

5) As oestrogen builds up in the blood stream, the brain cries out 'TOO MUCH!' and stops releasing FSH

6) However, the oestrogen hijacks the brain and forces the pituitary to release a new hormone, called Luteinising Hormone
, or LH (literally the 'Yellow-making hormone', because it makes granulosa cells turn yellow).

7) Luteinising hormone alters the follicles. Firstly, it can force the human egg to come out of dormancy, especially in the dominant follicle (human eggs are incompletely formed until right before egg release/ovulation). Secondly, it alters the functioning of the egg shell/granulosa: the granulosa starts secreting fluid into the follicle, causing the follicle to swell up into a large cyst. Within this cyst fluid, the egg floats about.

(The swelling up of the follicles often causes pain, which is often referred to as 'mid-cycle pain'. It is very common for women to experience abdominal pains/cramps just before the ovulation, because of these swollen cysts. The pain of these ovarian cysts can sometimes be so severe that it may necessitate and operation to remove the cysts.)

8) At some point, one of the mature follicles decides that it just can't take it anymore, and it bursts, releasing its egg into the wild environs of the female reproductive system. With a bit of luck, this egg will find its way into the uterine tubes, where it may, if destined, meet with a mighty sperm.

9) By now, Oestrogen levels are dropping (because the brain is on strike and refusing to make anymore FSH). With the drop of oestrogen, the levels of LH start to fall as well.

10) The remains of the egg shell, the granulosa, now transform into a new structure, the corpus luteim (literally, the 'yellow structure'). The corpus luteim does something new - it makes progesterone.

11) At this point oestrogen levels, FSH and LH levels are low. The brain is refusing to make either FSH or LH because it is happy that there is progesterone in the blood stream and no more female hormones are required. No oestrogen is made because the follicles fall asleep again. Only the corpus luteum is busy chugging along, pumping out progesterone.

12) However, eventually the corpus luteum exhausts itself. The structure shrivels up and dies. All the cells basically kill themselves off in a mad flurry of progesterone production. They push themselves so hard that they die.

13) With the death of the corpus luteum, progesterone levels fall. Oestrogen levels have been low for a while by now too. The brain freaks out and tells the pituitary gland to activate, starting the whole cycle anew.

This cycle can be interrupted by pregnancy, as the early pregnancy makes its own progesterone; or by hormonal contraception, which prevents the release of FSH and therefore prevents the maturation of egg cells and their granulosa shells.

Now let's go onto the most obvious manifestation of the female reproductive cycle, the Menstrual Cycle:

Menstruation, by definition, is blood loss from the uterus that occurs as part of the normal female reproductive cycle.

Menstruation remains a medical mystery. It is unclear as to why human females menstruate. The vast majority of mammals do not menstruate. In fact, only some primates, some bats, and the elephant shrew have menstrual cycles (of interest, is that menstruation seems to have developed independently in these three groups i.e. there is no menstruating ancestor held in common). Some mammals (eg dogs) do have something called an oestrus cycle, but this causes vaginal bleeding, not menstrual bleeding.

Menstrual bleeding causes iron deficiency, anemia, low blood volume, and can predispose to nasty infections. It appears, at first glance, to be evolutionarily disadvantageous - women routinely can become severely ill due to their menstrual cycle. Yet, it must have some evolutionary advantage to have occurred, but this is a different discussion that I will not go into in this post.

Now we have already established that there is an ovarian cycle, and that it release oestrogen before the egg is released, and then progesterone after the egg is released. The uterus responds to each of these hormones.

Oestrogen causes the inner lining, the endometrium, of the uterus to grow and become thick and cushiony. You could say that oestrogen prepares the uterus to receive the egg, by making a pleasant environment for the egg to fertilise in. When oestrogen levels fall, progesterone takes over.

Progesterone is a maintainer - it doesn't cause the endometrium to grow, but it does prevent it from decaying. It also causes the endometrium to be rich with blood and nutrient, in case a fertilised egg comes along and needs to grab a snack.

So what happens when both oestrogen and progesterone levels fall, which is what occurs as the corpus luteum dies?

Well, then there is nothing to grow the endometrium and there is nothing to maintain the endometrium.

So it dies off. This process can be painful and causes pre-menstrual cramps, which is literally the feeling of blood flow being cut off to the endometrium (similar to the kind of pain you experience if you run and your muscles just can't get enough blood and you start cramping).

As the endometrium dies off, it tears the blood vessels embedded within it. These blood vessels bleed into the uterus.

Blood accumulates and then the combined blood-dead tissue mix pour out of the uterus, via the vagina, as a period.

Once oestrogen levels build up again, then the uterine lining heals itself and starts growing again, and the period finally stops. Periods can also be stopped by giving a women progesterone or oestrogen, for example with hormonal contraceptives.

A very brief note on the vaginal and cervical cyles:
Oestrogen makes the vagina and cervix wet and moist, with secretions that provide sperm with nutrition. Therefore, women are more likely to be wet just before ovulation, due to the effects of oestrogen.

Progesterone assumes that the egg has been fertilised already and it tries to prevent further sperm coming in. The vagina and cervix become dry and have a thick, sticky secretion that is full of white blood cells that will try to destroy sperm. Women can be very dry after ovulation.

Congratulations if you made it this far. You now know more than most women know about the female reproductive cycle. Any questions?