Putting it all Together: The Menstrual Cycle Hormones

 

The hormones of the menstrual cycle are more complicated than the anatomy. I never understood this until medical school, and honestly I think we still have a ways to go in terms of scientific research on it. It all comes down to fluctuating hormone levels and ovarian function. 

 

The following chart summarizes the hormones and their effects in the ovaries (and then I'll go into more detail below):

 

Some important terms before we get started: 

Anterior pituitary hormones = Hormones made in the brain which are released through the bloodstream to reach the ovaries (eg: FSH & LH).

Ovarian hormones = Hormones made in the ovaries which are released through the bloodstream to reach the brain (eg: estrogen & progesterone).

Uterine cycle = The state of the endometrial lining in the uterus (the cushion that builds up each month and then gets shed out).

Follicle, aka ovarian follicle = The house for the egg within the ovary; it helps nourish the egg and grow it until it is released at ovulation. Eggs are never free-floating in the ovary; they are all inside follicles. 

Primordial follicle = Basically an egg but it has a small coating of cells around it. This is the storage form of the eggs. 

 

Ok now let's get started and try to clarify some of this! We are going to assume a regular, 28-day cycle here. 

 

Day 0

First day of the menstrual period. Inside the ovary, follicles are starting to get built around a handful of eggs. The rest of the thousands of eggs (in primordial follicle form) stay dormant; they might get selected next month. The body temperature stays low. The brain is sending out low levels of FSH and LH. FSH is called "Follicle Stimulating Hormone" because it does just that-- it sends the message for follicles to start growing around a handful of eggs. We do not know exactly why certain eggs are picked to grow each month, but it relates to their reception of chemical messengers from the brain.

 

Days 5-6

The menstrual period starts to end and the uterus is down to its thinnest amount of endometrial lining. Inside the ovary, the growing follicles continue to get larger and nourish their eggs inside (due to receiving FSH from the brain). These growing follicles secrete estrogen, which travels through the bloodstream and back up to the brain. One of the follicles wins out and becomes the "dominant follicle" and receives the majority of the nourishment. We do not know exactly why one out-competes all of the others for the month, but we know it happens (except for in cases of non-identical twins!). It will become the follicle that releases the ovulated egg. Body temperature stays low. 

 

Days 10-13

Estrogen secretion continues to climb to a very high level. LH begins to surge. FSH also increases, but not as much. By this point the dominant follicle is almost ready to release its egg for ovulation. The endometrial lining of the uterus is thickening due to the estrogen it receives through the bloodstream. 

 

Day 14

The peak of the surge in LH induces ovulation, and the egg ruptures out of the ovary wall and gets sucked up into the fallopian tube. Body temperature peaks at ovulation and stays high afterwards. The egg travels up the fallopian tube and stays at the top part for 24 hours. If fertilized, it continues to migrate into the uterus to implant and become a fetus. If not fertilized within 24 hours, it disintegrates. For the purpose of this site I will not go into pregnancy, because, let's face it: if you're visiting my site you probably don't want to read about that! 

 

Days 15-22

Progesterone is produced by the remnant of the dominant follicle in the ovary (now empty) and its level peaks around day 21 and then starts to drop. estrogen during this time also climbs and drops in a similar fashion. The endometrial lining continues to build up to high levels under the influence of progesterone (progesterone would be high during early pregnancy as well, so the uterus essentially doesn't know yet if it should be preparing for an embryo to implant). 

 

Day 28

The drop in progesterone signals to the uterus that there is no pregnancy, and therefore it is time to shed off the uterine lining. This is the beginning of menstruation. Body temperature decreases to baseline level again. And the cycle restarts...