What Is Menstruation?
Menstruation refers to the process in which the endometrium (uterine lining)—which has thickened in preparation for pregnancy—breaks down and is discharged from the body due to the absence of the hormone hCG and a drop in progesterone levels.
There are four main hormones involved in the menstrual process:
LH (Luteinizing Hormone) and FSH (Follicle-Stimulating Hormone) secreted by the pituitary gland, and estrogen and progesterone secreted by the ovaries.
Oocytes and Menopause
In males, sperm are continuously produced in the seminiferous tubules, but females are born with a limited number of eggs (oocytes) that last throughout life. Once these eggs are completely used up, a woman officially enters menopause.
In the beginning, these eggs remain in the ovaries as immature oocytes.
STEP 1. FSH and Follicle Growth
After menstruation ends, FSH secreted by the pituitary gland stimulates the growth of immature follicles into mature ones. You can think of a follicle as a tiny sac that encloses an egg.
As the follicle matures, a particularly dominant one—called the dominant follicle—begins to secrete estrogen. This estrogen reduces the secretion of FSH and stimulates the rebuilding of the uterine lining. At the same time, it triggers the release of LH, a hormone directly associated with ovulation.
🩸 The Role of the Endometrium
The uterine lining is built layer by layer with capillary networks
to ensure efficient exchange of materials between the mother and the embryo in case pregnancy occurs.
STEP 2. Ovulation
As ovulation approaches, LH levels peak.
The mature follicle—the sac surrounding the egg—ruptures,
and the egg inside finally comes out, as if saying “peek-a-boo!”
This is what we know as ovulation.
STEP 3. The Luteal Phase and Progesterone
After ovulation, LH levels drop sharply while estrogen stays at a moderate level. The thickened uterine lining is now supported by progesterone, which prepares the body for possible pregnancy.
This progesterone is secreted by the corpus luteum, which is basically the remnant shell of the follicle after ovulation. The corpus luteum works like a steady hand, holding the heavy capillary walls of the endometrium firmly in place.
STEP 4. The Start of Menstruation
However, if pregnancy does not occur, the corpus luteum begins to degenerate.
In other words, it can no longer hold the uterine lining together.
Eventually, the capillary tissues of the endometrium collapse and are expelled from the body.
Those so-called “clots” or “chunks” people often talk about are actually these tissue fragments.
This entire process is what we call menstruation.
Q. Why Menstruation Stops During Pregnancy?
You might now wonder: why does menstruation stop when a woman becomes pregnant?
When fertilization occurs and the embryo implants into the uterine wall, it begins to secrete a hormone called hCG (human Chorionic Gonadotropin). This hormone plays a crucial role in early implantation and placental formation. It stimulates the degenerating corpus luteum, allowing it to continue producing progesterone.
Thanks to this progesterone, the uterine lining remains intact— there’s no reason for it to shed, so menstruation does not occur.
Interestingly, hCG can be detected both in urine and blood, which is why the pregnancy test kit (“pregnancy stick”) we often use works by detecting the presence of this hormone.
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