Periods happen because your body prepares for pregnancy each month, and when pregnancy doesn’t occur, it sheds the tissue it built up. The uterus grows a thick, blood-rich lining to support a fertilized egg. If no egg implants, hormone levels drop, the lining breaks down, and it exits the body as menstrual blood. This cycle repeats roughly every 21 to 35 days from puberty (around age 13 or 14 on average) until menopause (around age 50).
The Monthly Buildup
The uterine lining, called the endometrium, has two layers. The deeper layer is permanent and stays in place throughout your life. The surface layer is temporary. It’s the one that thickens each cycle and sheds during your period, then regenerates from the deeper layer all over again.
The whole process is driven by hormones produced in a feedback loop between your brain and your ovaries. In the first half of the cycle, your pituitary gland (a small structure at the base of the brain) releases a hormone called FSH, which tells a group of follicles in the ovaries to start developing. These follicles produce rising levels of estrogen, which signals the uterine lining to grow thicker and develop new blood vessels. By mid-cycle, the lining has become a soft, nutrient-rich layer ready to receive a fertilized egg.
Ovulation: The Turning Point
Around the middle of the cycle, typically 34 to 36 hours before an egg is released, the brain triggers a surge of another hormone called LH. This surge causes the most mature follicle in the ovary to release its egg. That event is ovulation.
After the egg is released, the empty follicle transforms into a temporary structure that starts pumping out progesterone. Progesterone is the hormone that stabilizes and maintains the uterine lining. It keeps the lining in place, further develops its blood supply, and essentially holds everything together in case a fertilized egg arrives. Think of progesterone as the signal that says “stay ready.”
What Triggers the Bleeding
If the egg isn’t fertilized within about 24 hours, it dissolves. Without a pregnancy signal, the structure producing progesterone begins to break down over the next 10 to 14 days. Progesterone levels fall sharply, and that drop is the direct trigger for your period.
When progesterone drops, the small spiral-shaped blood vessels in the uterine lining begin to constrict and coil tightly. This cuts off blood flow to the surface layer of the lining, starving the tissue of oxygen. The oxygen-deprived tissue starts to break apart. At the same time, the endometrium releases chemical signals called prostaglandins, which do two things: they cause the muscular wall of the uterus to contract (pushing the tissue out) and they further constrict blood vessels in the lining. The combination of tissue breakdown, muscle contractions, and blood vessel disruption produces menstrual bleeding.
Once the surface layer has been shed, a muscular segment near the base of those spiral blood vessels acts like a natural valve, constricting to slow and eventually stop the bleeding. The deeper permanent layer of the endometrium then begins regenerating the surface layer, and the whole cycle starts over.
Why Periods Hurt
The prostaglandins that help shed the lining are also responsible for period pain. They increase the force and frequency of uterine contractions, and when the contractions are strong enough, they temporarily reduce blood flow to the uterine muscle itself. That combination of intense squeezing and reduced blood flow creates the cramping sensation most people recognize as period pain. People who produce higher levels of prostaglandins tend to experience more severe cramps, which is why anti-inflammatory pain relievers (which block prostaglandin production) are often the most effective treatment.
How Much Blood You Actually Lose
A typical period lasts 2 to 7 days, and the total blood loss is usually around 60 milliliters, or roughly 4 tablespoons. That’s less than most people expect. Menstrual fluid also contains tissue and mucus, so the total volume coming out appears larger than the actual blood loss. Losing more than 80 milliliters per cycle is considered heavy bleeding, which may show up as needing to change a pad or tampon every hour for several hours, passing large clots, or bleeding through clothing regularly.
The Bigger Picture: Puberty to Menopause
Periods typically begin around age 13 or 14, though anywhere from 9 to 16 is within the normal range. In the first year or two, cycles are often irregular because the hormonal feedback loop between the brain and ovaries is still maturing. Ovulation doesn’t happen every cycle at first, which means some early periods are lighter or unpredictable.
For most of adulthood, cycles settle into a pattern that repeats every 21 to 35 days. Stress, significant weight changes, intense exercise, and certain health conditions can all disrupt the hormonal signals and cause irregular or missed periods without pregnancy being involved. Hormonal birth control works by overriding the natural cycle, typically by preventing ovulation or keeping progesterone levels steady so the lining doesn’t build up as much.
Periods stop permanently at menopause, which occurs around age 50 on average. In the years leading up to menopause (a phase called perimenopause), the ovaries gradually produce less estrogen and progesterone. Cycles become irregular, periods may get heavier or lighter, and eventually the hormonal signals are no longer strong enough to build and shed the lining at all.