Women get periods because the reproductive system regularly prepares for a possible pregnancy. Each menstrual cycle, hormones coordinate changes in the ovaries and uterus. An egg develops and is released from an ovary, while the lining of the uterus thickens to create a suitable environment for a fertilized egg.
If pregnancy does not occur, the hormone signals that maintain that uterine lining decline. The lining then breaks down and leaves the body through the vagina as menstrual bleeding, or a period.
A period is therefore not the body “getting rid of an egg.” It is mainly the shedding of the uterine lining that was prepared for a possible pregnancy.
What happens during the menstrual cycle?
The menstrual cycle is a repeating sequence of hormonal and physical changes involving the brain, ovaries, and uterus. Although cycles are often described as lasting about 28 days, normal cycles can be shorter or longer, and their timing can vary between people and from one cycle to another.
The cycle is commonly divided into four overlapping stages: menstruation, the follicular phase, ovulation, and the luteal phase.
Menstruation begins when the uterine lining is shed. At the same time, hormonal changes stimulate the ovaries to begin developing follicles, small structures that contain immature eggs.
During the follicular phase, one follicle typically becomes dominant and continues developing. It produces increasing amounts of estrogen. Estrogen helps rebuild and thicken the uterine lining after menstruation.
Around the middle of the cycle, a sharp rise in luteinizing hormone, or LH, triggers ovulation. The mature follicle releases an egg from the ovary.
After ovulation, the emptied follicle becomes a temporary hormone-producing structure called the corpus luteum. It produces progesterone, along with some estrogen. Progesterone helps maintain and further prepare the uterine lining in case a fertilized egg implants.
If fertilization and pregnancy do not occur, the corpus luteum eventually breaks down. Progesterone and estrogen levels fall, the uterine lining loses the hormonal support it needs, and menstruation begins.
Why does the uterus build up a lining?
The uterus needs a specialized lining because a pregnancy requires a place where an embryo can implant and receive support.
The inner lining of the uterus is called the endometrium. Throughout the menstrual cycle, hormones cause this tissue to change. It becomes thicker and develops the blood vessels and glands needed to support a potential pregnancy.
This preparation happens even though pregnancy may not occur in that particular cycle.
If an embryo implants successfully, hormonal signals associated with pregnancy maintain the uterine lining. If there is no pregnancy, those signals are absent, and the hormonal support provided by the corpus luteum declines. The endometrium then undergoes changes that lead to menstrual bleeding.
What actually causes menstrual bleeding?
Menstrual bleeding begins when estrogen and progesterone levels fall at the end of a cycle without pregnancy.
The drop in progesterone is particularly important. It causes blood vessels supplying the functional portion of the uterine lining to constrict and the tissue to break down. Inflammatory and biochemical processes then contribute to the separation of the lining from the uterine wall.
The blood, mucus, fluid, and tissue from the uterine lining pass through the cervix and out through the vagina.
Menstrual fluid is therefore not simply blood. It contains blood along with tissue and secretions from the reproductive tract.
Why does the body go through this cycle?
The menstrual cycle is part of the reproductive system’s way of coordinating ovulation and preparing the uterus for pregnancy.
The ovaries and uterus operate as a connected system. Hormones produced by the brain regulate the ovaries, while hormones from the ovaries control major changes in the uterus.
The process begins in the brain. The hypothalamus releases gonadotropin-releasing hormone, or GnRH, which stimulates the pituitary gland to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These hormones act on the ovaries and help control follicle development and ovulation.
The ovaries then produce estrogen and progesterone, which act on tissues throughout the reproductive system and also feed information back to the brain. This hormonal communication allows the timing of follicle development, ovulation, and uterine preparation to be coordinated.
Why does the lining shed if there is no pregnancy?
The uterine lining is maintained by hormonal signals rather than simply remaining in place indefinitely.
After ovulation, progesterone from the corpus luteum helps keep the endometrium in a state suitable for implantation. If pregnancy does not occur, the corpus luteum stops functioning. As a result, progesterone and estrogen levels fall.
Without sufficient hormonal support, the upper functional layer of the endometrium breaks down and is shed. The body then begins another cycle of developing an egg and rebuilding the uterine lining.
This is why menstruation is closely tied to the hormonal events following ovulation.
Is a period the body getting rid of an unfertilized egg?
No. The unfertilized egg and the menstrual lining are different things.
After ovulation, an egg travels into the fallopian tube. If sperm does not fertilize it, the egg survives only for a limited period before degenerating. Menstruation occurs later as the uterine lining is shed.
The egg itself is extremely small and does not account for menstrual blood. The visible material during a period primarily comes from the endometrium and associated blood and secretions.
Why do periods sometimes hurt?
Menstrual cramps are largely caused by substances called prostaglandins.
When menstruation begins, the uterine lining produces prostaglandins that promote contractions of the uterine muscle. These contractions help the uterus shed its lining. Higher prostaglandin activity can produce stronger contractions and more pain.
That pain may be felt in the lower abdomen and can sometimes spread to the lower back or thighs. Some people also experience symptoms such as nausea, diarrhea, headaches, or fatigue around menstruation.
Mild to moderate discomfort can be a normal part of menstruation, but severe or worsening menstrual pain is not something that should automatically be considered normal. Conditions such as endometriosis, adenomyosis, or uterine fibroids can cause significant menstrual pain and may require medical evaluation.
Why don’t periods happen during pregnancy?
During pregnancy, the hormonal environment changes substantially.
After implantation, pregnancy-related hormonal signals maintain the corpus luteum early in pregnancy, allowing it to continue producing progesterone. Later, the placenta becomes the major source of progesterone and other hormones that help maintain the uterine lining.
Because the endometrium is being maintained rather than shed, normal menstruation does not occur during pregnancy.
Bleeding during pregnancy can happen for other reasons, however, and should not automatically be interpreted as a period.
Why do periods stop after menopause?
Menstruation eventually stops because ovarian reproductive function changes with age.
Before menopause, the ovaries contain follicles that can develop eggs and produce reproductive hormones. Over time, the number and responsiveness of these follicles decline. Ovulation becomes less predictable, hormone production changes, and menstrual cycles become increasingly irregular.
Eventually, the ovaries no longer release eggs regularly, and the hormonal cycling that produces menstrual periods ends. Menopause is reached after a person has gone 12 consecutive months without a menstrual period, assuming there is no other explanation for the absence of menstruation.
Why do periods begin at puberty?
Periods begin when the reproductive system becomes hormonally active during puberty.
The hypothalamus begins releasing GnRH in a pattern that stimulates the pituitary gland to produce FSH and LH. These hormones activate the ovaries, which begin producing increasing amounts of estrogen and eventually establish regular cycles of follicle development, ovulation, and changes in the uterine lining.
Early menstrual cycles are often irregular because the hormonal system is still developing its mature pattern of coordination. Regularity can change throughout reproductive life for many reasons, including puberty, pregnancy, breastfeeding, hormonal contraception, perimenopause, and other changes affecting the reproductive system.
