Understanding the biology of your menstrual cycle

Virtually everything related to women’s health comes back to our menstrual cycle. If you’re interested in a deeper dive into the biology, read on.

This is a simplified version of the cycle.

Normal Menstrual Cycle
Illustration by iStock Photo

Before we head toward the pelvic, uterus and ovaries, here’s a drawing I made that shows the influence of the hypothalamus and the pituitary gland in the brain.

Glossary of terms:

ACTH: Adrenocortiotropin Hormone: Acts at the Adrenal glands
FSH: Follicle Stimulating Hormone: works at the ovary to stimulate the production of follicles, which contain the eggs
GNRH: Gonadatropin Releasing Hormone
Gonadatropins: are hormones that are specific to the gonads (ovaries and testes)
LH: Luteinizing Hormone: works at the ovary to stimulate ovulation
Prolactin: works at the breast to stimulate milk production
TSH: Thyroid Stimulating Hormone

The Biology of the Menstrual Cycle:

When a woman starts her period, the hypothalamus and pituitary are already working to start a new cycle.
The hypothalamus sends Gonadatropin Releasing Hormone (GNRH) to the pituitary to stimulate the release of FSH, Follicle Stimulating hormone.
The ovary will respond to FSH by helping follicles, each contain an egg and are small and fluid filled, to grow
As the follicles grow, they produce Estrogen, the hormone that causes breast development, women to have curves, softer skin, higher pitched voice, etc.
Estrogen also stimulates the lining of the uterus to grow with lots of tissue and blood in preparation for the possibility of a fertilized egg.
It’s kind of a race, with one of the follicles growing much faster than the others, producing the most estrogen, in fact the estrogen level will peak.
This superstar follicle becomes known as the dominant follicle, which will ultimately release the winning egg.
When the pituitary receives information from the feedback loops that the estrogen level has peaked, it will release LH, Luteininizing Hormone.
LH stimulates the final maturation of the egg. It will also peak in value and it prepares the ovary to start producing Progesterone
The peak of LH is what women check for when they use Ovulation Prediction Kits (OPK)
When ovulation occurs, the follicle changes it’s name becomes known as the corpus luteum: get it? Luteinizing hormone: Corpus Luteum.?
The corpus luteum produces a lot of Progesterone which causes the lining of the uterus to stop growing in height, and to become thicker and more glandular in preparation for the possibility of a fertilized egg
After about 10 to 14 days, the corpus luteum shrinks and thus both the Estrogen and Progesterone levels drop signaling to the uterine lining to let go, there’s no pregnancy and to have a period.
Progesterone balances the Estrogen and prevents uncontrolled growth of the uterine lining. Women who don’t ovulate don’t produce enough progesterone to prevent irregular and heavy bleeding.
In pregnancy, the fertilized egg nestles into the uterine lining sending signals in the form of Human Chorionic Gonadatropin (HCG) to the uterine lining and the pituitary to signal to inhibit or stop the cycling process.

The Quickest Explanation

A period starts
Hormone levels drop
A new period starts

So, now you know! I hope this helps and please feel free to share this with anyone in a biology class who needs the simplified explanation.

Understanding this will help you understand:

Fertility Concerns

  • Why the birth control pill is made of Estrogen and Progesterone
  • Why women who don’t ovulate have irregular and heavy cycles
  • Why Estrogen levels drop at menopause, when there are no more eggs

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