The menstrual cycle has four distinct phases that repeat roughly every 24 to 38 days. While 28 days is often cited as the “textbook” cycle, most people fall somewhere within that wider range, and both ends are completely normal. Each phase is driven by shifting hormone levels that prepare the body for a possible pregnancy, then reset when pregnancy doesn’t occur.
How the Cycle Is Counted
Day 1 of your cycle is the first day of your period, not the last. The cycle ends the day before your next period starts. So if you get your period on March 1 and your next period arrives on March 29, that cycle was 28 days long. Tracking from period to period is the simplest way to learn your personal pattern.
Your cycles are considered regular as long as they consistently fall between 24 and 38 days. They don’t need to be exactly the same length every month. If your cycles regularly fall outside that window, or if you go 90 days without a period (and you’re not pregnant or breastfeeding), that’s worth bringing up with a healthcare provider.
Phase 1: Menstruation
Menstruation is the shedding of the uterine lining that built up during the previous cycle. When pregnancy doesn’t happen, levels of both estrogen and progesterone drop sharply. Without those hormones to maintain it, the thickened inner layer of the uterus breaks down and exits through the vagina as menstrual blood, tissue, and mucus.
A period typically lasts anywhere from two to seven days. The total amount of blood lost varies widely from person to person. Losing more than about 80 milliliters (roughly five to six tablespoons) per cycle is the traditional threshold for “heavy” bleeding, though in practice what matters more is whether your flow interferes with daily life or causes symptoms like fatigue or lightheadedness.
Phase 2: The Follicular Phase
The follicular phase overlaps with menstruation and extends beyond it. It starts on day 1 of your period and lasts until ovulation, making it the longest and most variable part of the cycle. If your cycles tend to run longer or shorter than average, the follicular phase is almost always the reason.
Here’s what happens: the pituitary gland, a small structure at the base of the brain, releases follicle-stimulating hormone (FSH). This hormone signals the ovaries to start developing several small fluid-filled sacs called follicles, each containing an immature egg. Over the next week or so, one follicle outpaces the rest and becomes the dominant follicle. The others stop growing.
As the dominant follicle matures, it produces rising amounts of estrogen. That estrogen does two important things. First, it tells the uterus to start rebuilding its lining, creating a fresh, blood-rich layer in case a fertilized egg needs to implant. Second, once estrogen reaches a high enough level, it triggers a surge of luteinizing hormone (LH) from the pituitary gland. That LH surge is the signal that ends the follicular phase and kicks off ovulation.
Phase 3: Ovulation
Ovulation is the shortest phase, lasting roughly 24 hours. It begins when the LH surge causes the dominant follicle to rupture and release a mature egg from the surface of the ovary. This typically happens 24 to 48 hours after the LH surge begins, or more precisely, 8 to 20 hours after LH hits its peak.
Once released, the egg is swept into the fallopian tube, where it can potentially be fertilized by sperm. The egg survives only 12 to 24 hours after ovulation. Sperm, on the other hand, can live inside the reproductive tract for up to five days. This is why the fertile window extends several days before ovulation and closes shortly after it.
For people with a 28-day cycle, ovulation often occurs around day 14, but this is only an estimate. If your cycle is shorter or longer, ovulation shifts accordingly. Ovulation predictor kits detect the LH surge in urine, giving a heads-up roughly one to two days before the egg is released.
Phase 4: The Luteal Phase
After the egg leaves, the empty follicle doesn’t just disappear. It transforms into a temporary structure called the corpus luteum, which pumps out progesterone along with some estrogen. Progesterone is the hormone that makes the uterine lining soft, spongy, and receptive to a fertilized egg. It also slightly raises your resting body temperature, typically by 0.4 to 1.0 degrees Fahrenheit, which is why temperature tracking can help confirm that ovulation has already occurred.
The luteal phase is more consistent than the follicular phase, lasting about 10 to 16 days in most people. If the egg isn’t fertilized, the corpus luteum begins to break down roughly 10 days after ovulation. As it degrades, progesterone and estrogen levels fall. Without those hormones supporting it, the uterine lining can no longer sustain itself, and menstruation begins. The cycle resets.
If the egg is fertilized and implants successfully, the early pregnancy produces a hormone (hCG) that keeps the corpus luteum alive and producing progesterone. This is the same hormone that pregnancy tests detect.
Physical Signs Throughout the Cycle
Your body gives several clues about where you are in the cycle, even without a test or a tracker. One of the most noticeable is cervical mucus, the discharge you may see on underwear or toilet paper.
- During and just after your period (roughly days 1 to 4): discharge is dry or tacky, often white or slightly yellow.
- Mid-follicular phase (days 4 to 9): it shifts to sticky, then creamy and yogurt-like, with a cloudy white appearance.
- Approaching ovulation (days 10 to 14): mucus becomes clear, slippery, and stretchy, resembling raw egg whites. This is your most fertile window.
- After ovulation through the luteal phase (days 15 to 28): discharge dries up again and stays minimal until your period arrives.
Basal body temperature follows a different pattern. Before ovulation, your resting temperature stays in a lower range. After ovulation, the progesterone released by the corpus luteum nudges it up by less than half a degree Fahrenheit in many people, though the increase can be as much as a full degree. The temperature stays elevated throughout the luteal phase and drops back down when your period starts (or stays elevated if you’re pregnant).
Why Cycles Vary in Length
Almost all normal variation in cycle length comes from the follicular phase. Some months, the dominant follicle develops quickly; other months, it takes longer. The luteal phase, by contrast, stays relatively fixed for each individual. This means that if you have a 32-day cycle one month and a 26-day cycle the next, ovulation simply happened earlier or later.
Factors like significant weight changes, intense exercise, illness, travel, and major life transitions can all influence how quickly (or slowly) the follicular phase unfolds. Interestingly, while everyday psychological stress is often blamed for irregular periods, research has not found a consistent link between general stress levels and abnormal cycle length. Severe or prolonged stress can suppress the hormonal signals from the pituitary gland, but ordinary daily stress is less likely to throw off your cycle than many people assume.
Cycles also tend to be more irregular at the extremes of reproductive life. In the first few years after periods begin, and again in the years leading up to menopause, it’s common for cycles to fall outside the 24-to-38-day range as the hormonal system ramps up or winds down.