Periods happen because your body prepares for pregnancy every cycle, and when pregnancy doesn’t occur, it sheds the preparation it no longer needs. Specifically, the lining of your uterus thickens each month to create a soft, blood-rich environment where a fertilized egg could implant and grow. When no egg implants, your hormone levels drop, and that thickened lining breaks down and exits your body as menstrual fluid. It’s a reset, clearing the slate so your body can build a fresh lining for the next cycle.
What Happens Inside Your Uterus Each Cycle
Your uterus has an inner lining called the endometrium, and it goes through dramatic changes every single cycle. In the first half of your cycle, after your period ends, hormones signal the endometrium to start growing. It thickens from as thin as 1 to 4 millimeters during your period to about 12 to 13 millimeters before ovulation. After you ovulate (release an egg from an ovary), the lining keeps building, reaching 16 to 18 millimeters just before your next period. That’s roughly half an inch of spongy, blood-vessel-rich tissue designed to nourish a potential pregnancy.
The endometrium has two layers. The deeper base layer stays put throughout your cycle. The upper layer, called the functional layer, is the one that changes. It’s where a fertilized egg would implant, and it’s what sheds during your period. When it breaks down, the fluid that leaves your body is a mix of endometrial cells and blood from the small blood vessels that fed that lining.
The Hormones That Trigger Bleeding
Two hormones run the show: estrogen and progesterone. Estrogen drives the thickening of the lining in the first half of your cycle. After ovulation, progesterone takes over, stabilizing and enriching the lining so it’s ready for an embryo. Think of progesterone as the hormone holding everything in place.
If no fertilized egg implants within about two weeks of ovulation, your ovaries stop producing high levels of both estrogen and progesterone. That hormone drop is the direct trigger for your period. Without progesterone to maintain it, the thickened lining loses its structural support. Small arteries in the lining constrict and cut off blood flow to the upper layer, causing the tissue to break down. The lining detaches, and your body pushes it out through the cervix and vagina over several days.
Those same constricting arteries also act like a built-in shutoff valve. After the shedding is complete, they help stop the bleeding so your body can start building the lining again from scratch.
Why the Body Doesn’t Just Keep the Lining
It seems wasteful to build up tissue every month only to discard it. Biologists have explored several theories for why menstruation evolved rather than the body simply reabsorbing the lining, which is what most mammals do.
One long-standing theory is energy conservation. Maintaining a thick, metabolically active uterine lining all the time would cost the body more energy than shedding and rebuilding it on a cyclical basis. It’s more efficient to keep the lining in its low-energy, thin state and only invest in building it up when there’s a real chance of pregnancy that cycle.
A more recent theory focuses on a process called spontaneous decidualization. In humans and other menstruating species, the uterine lining transforms on its own every cycle, regardless of whether an embryo is present. This transformation gives the body a way to screen embryos before allowing deep implantation. Human embryos are unusually aggressive implanters compared to those of most mammals. The decidualized lining acts as a protective barrier, letting the mother’s body evaluate embryo quality and reject those that are abnormal or unlikely to survive. Shedding the lining is a natural extension of this screening process.
An older hypothesis suggests that menstruation helps clear bacteria and other infectious agents that sperm may carry into the uterus. While this idea has been largely supplanted by the decidualization theory, it likely played some role in the evolutionary picture.
What a Normal Period Looks Like
A typical menstrual cycle lasts between 24 and 38 days, measured from the first day of one period to the first day of the next. The often-cited 28-day cycle is just an average, and plenty of people fall on either side of it. Your periods are considered regular as long as they consistently land within that 24-to-38-day window.
Most periods last between three and seven days. Bleeding that regularly stretches beyond eight days is considered heavy menstrual bleeding and is worth discussing with a healthcare provider. In terms of volume, normal blood loss during a period is generally under 80 milliliters for the entire period, which is roughly five to six tablespoons. That number surprises many people because periods can feel much heavier than they actually are, partly because menstrual fluid isn’t pure blood. It’s a combination of blood, tissue from the uterine lining, and mucus.
Bleeding Without Ovulating
Technically, menstruation is the shedding that follows an ovulatory cycle. But you can experience bleeding even in cycles when you don’t release an egg. This is called anovulatory bleeding, and it happens because estrogen still stimulates the lining to grow. Without ovulation, though, there’s no progesterone surge to stabilize the lining properly. Eventually the lining outgrows its own support and sheds irregularly.
Anovulatory cycles are common during puberty, the years leading up to menopause, and in conditions like polycystic ovary syndrome. The bleeding can be unpredictable in timing and flow, sometimes heavier or lighter than a typical period. While the occasional anovulatory cycle is normal, frequent ones can signal a hormonal imbalance worth investigating.
Why Only Some Species Menstruate
Menstruation is rare in the animal kingdom. Besides humans, only a handful of primates, some bat species, and the elephant shrew are known to menstruate. The common thread is spontaneous decidualization. In these species, the uterine lining transforms every cycle whether or not an embryo arrives, and shedding is the natural consequence when one doesn’t.
Most other mammals handle things differently. Their uterine lining only transforms in response to an actual embryo, so there’s no buildup to shed when pregnancy doesn’t occur. The lining simply reabsorbs. The fact that menstruation evolved independently in several unrelated species suggests it offers a meaningful reproductive advantage, most likely related to the ability to control which embryos successfully implant.