Periods exist because your body prepares for pregnancy every cycle, then clears the preparation when pregnancy doesn’t happen. Each month, the uterus builds a thick, blood-rich lining designed to nourish a fertilized egg. If no egg implants, hormone levels drop, and that lining sheds. The bleeding you experience is the body resetting itself to start the process over again.
What the Uterus Does Each Cycle
The uterine lining, called the endometrium, has one job: create a hospitable environment for an embryo. After ovulation (when an egg is released from the ovary), the cells left behind in the ovary transform into a temporary structure that pumps out progesterone. This hormone signals the uterus to build a densely vascularized bed of tissue, rich with blood vessels and nutrients, essentially a landing pad for a fertilized egg.
If pregnancy occurs, that structure keeps producing progesterone for roughly 10 weeks, maintaining the lining until the placenta takes over. If no embryo implants, the structure degrades within about 14 days. Progesterone and estrogen levels plummet, and that drop is the direct trigger for your period.
How the Lining Actually Sheds
When progesterone withdraws, the spiral-shaped blood vessels feeding the upper layer of the uterine lining begin to constrict and coil tighter. Blood flow to the surface layers decreases, and the tissue starts to break down from lack of oxygen. The uterus then releases compounds that cause its muscular walls to contract, physically pushing the degraded tissue out. Those contractions are what you feel as cramps.
Remarkably, the body starts rebuilding before it’s even finished clearing out. Within two days of bleeding starting, rising estrogen from the next batch of developing egg follicles already begins regenerating the lining’s surface. Destruction and reconstruction overlap.
What Menstrual Fluid Actually Contains
Period blood isn’t the same as the blood in your veins. It’s a mixture of blood, shed endometrial tissue, cervical mucus, and vaginal secretions. Compared to venous blood, it has a higher proportion of tissue and water but lower levels of red blood cells, iron, and hemoglobin. It also lacks the clotting factors found in regular blood, which is why it flows freely rather than clotting the way a cut would. The total volume lost during a typical period is about 60 milliliters, roughly four tablespoons, though it often feels like more.
Why Humans Menstruate at All
Most mammals don’t have visible periods. Dogs, cats, cows, and the vast majority of other species simply reabsorb their uterine lining if pregnancy doesn’t occur. Overt menstruation is rare, shared mainly by humans, some other primates, and a handful of bat and rodent species. So why did humans evolve to bleed instead of reabsorb?
The leading scientific explanation points not to menstruation itself as the useful trait, but to the process that makes it inevitable. In most mammals, the uterine lining only transforms into its thickened, pregnancy-ready state in response to an actual embryo arriving. In humans, the lining transforms every single cycle regardless, driven by the mother’s own hormones rather than a fetal signal. This preemptive transformation likely evolved because of conflict between maternal and fetal interests: human embryos implant aggressively and deeply into the uterine wall, and a lining that’s already prepared can better regulate and protect against the inflammation and oxidative stress that comes with deep placentation. The tradeoff is that when pregnancy doesn’t happen, that thickened lining has to go somewhere. Shedding it is the cost of being prepared.
Normal Ranges for Cycle Length and Bleeding
A normal menstrual cycle lasts between 21 and 35 days, counted from the first day of one period to the first day of the next. The average is 28 days, but plenty of healthy cycles fall shorter or longer. Bleeding itself typically lasts three to seven days, with most people bleeding for three to five. Losing more than 80 milliliters per period is considered heavy bleeding and worth investigating.
In teens, cycles can range from 21 to 45 days as the hormonal system matures. Most people begin menstruating during adolescence and continue until menopause, which occurs between ages 45 and 55 for most women worldwide.
Your Cycle as a Health Signal
Medical organizations increasingly treat the menstrual cycle as a “fifth vital sign,” alongside blood pressure, heart rate, body temperature, and breathing rate. The logic is straightforward: menstruation depends on a complex chain of hormonal signals between the brain, pituitary gland, and ovaries. When something disrupts your broader health, whether it’s significant stress, nutritional deficiency, thyroid dysfunction, or other conditions, that hormonal chain is often one of the first things affected.
Irregular periods, unusually heavy or light bleeding, cycles consistently shorter than 21 days or longer than 35, and bleeding lasting more than eight days can all signal hormonal imbalances, gynecological conditions, or infections. Tracking your cycle length and flow over time gives you a baseline. Changes from that baseline are often the earliest clue that something in your body has shifted.