Your period cycle, or menstrual cycle, is the time from the first day of one period to the first day of your next period. The average cycle length across the adult population is 28 days, but anywhere from 21 to 35 days is considered normal. Each cycle moves through four distinct phases driven by shifting hormone levels, all building toward one goal: preparing your body for a possible pregnancy and resetting when one doesn’t occur.
The Four Phases of Your Cycle
A complete menstrual cycle has four phases that overlap and flow into each other. In a typical 28-day cycle, they break down roughly like this:
- Menstruation (days 1 to 5): Your uterine lining sheds, producing your period. Bleeding usually lasts about 4 to 5 days, and the total blood loss is small, around 2 to 3 tablespoons.
- Follicular phase (days 1 to 13): This phase actually starts on the same day as your period and runs until ovulation. Your body begins developing a new egg inside a fluid-filled sac called a follicle, and your uterine lining starts rebuilding.
- Ovulation (around day 14): A mature egg is released from the ovary. This is the shortest phase, lasting roughly 24 hours.
- Luteal phase (days 15 to 28): After the egg is released, the empty follicle transforms into a structure that pumps out hormones to thicken your uterine lining further. If the egg isn’t fertilized, hormone levels drop and the lining sheds again, starting a new cycle.
These day ranges are approximations based on a 28-day cycle. If your cycle is shorter or longer, ovulation and the other transitions shift accordingly.
How Hormones Drive Each Phase
Four hormones do most of the work. In the first half of your cycle, your brain releases follicle-stimulating hormone (FSH), which tells your ovaries to start growing an egg. As that egg develops inside its follicle, the follicle produces rising amounts of estrogen. By about day 7, estrogen levels in your blood climb significantly, and your uterine lining begins to thicken from its post-period minimum to around 5 to 7 millimeters.
When estrogen reaches a high enough level, usually around day 13, it triggers a sudden surge of luteinizing hormone (LH). This LH surge is the direct trigger for ovulation. The egg is released 28 to 36 hours after the surge begins.
After ovulation, the leftover follicle becomes what’s called the corpus luteum. Its main job is producing progesterone, the hormone that transforms your uterine lining into a welcoming environment for a fertilized egg. Progesterone dominates the second half of your cycle. If pregnancy doesn’t happen, the corpus luteum breaks down, progesterone and estrogen levels drop sharply, and your period begins.
What Your Body Tells You Along the Way
You don’t need a blood test to sense where you are in your cycle. One of the most reliable signals is cervical mucus, the discharge you notice on underwear or when wiping. It follows a predictable pattern in a 28-day cycle:
Right after your period ends, discharge tends to be dry or tacky. Over the next several days it becomes sticky and slightly damp, then transitions to a creamy, yogurt-like consistency. Around days 10 to 14, as you approach ovulation, it becomes slippery, stretchy, and clear, resembling raw egg whites. This wet, slippery mucus makes it easier for sperm to travel and signals your most fertile window. After ovulation, discharge dries up again and stays that way until your next period.
Other signs of ovulation can include mild one-sided pelvic cramping, a slight rise in body temperature (about half a degree), and increased sex drive. Not everyone notices these shifts, but tracking them over a few months can help you recognize your own pattern.
What Counts as a Normal Cycle
The 28-day cycle gets all the attention, but it’s just an average. Cycle length varies naturally by age, body weight, and ethnicity. People under 20 tend to have longer cycles, averaging about 30.3 days, compared to 28.7 days for those aged 35 to 39. Younger people also see more variation from month to month, with cycle lengths differing by an average of 5.3 days.
Some variation between cycles is completely normal. A cycle that’s 26 days one month and 30 days the next doesn’t necessarily signal a problem. What matters more is the overall pattern over several months.
When a Cycle Is Considered Irregular
Cycles fall outside the normal range when they’re consistently shorter than 21 days or longer than 35 days apart, when bleeding lasts longer than 7 days, or when the gap between cycles swings by more than 9 days (for example, 28 days one month, then 37 the next, then 29). Missing three or more periods in a row is considered abnormal unless you’re pregnant, breastfeeding, or approaching menopause.
Common causes of irregular cycles include stress, significant weight changes, starting a new exercise routine, and switching birth control methods. These are often temporary disruptions. Ongoing irregularity can point to conditions like polycystic ovary syndrome, endometriosis, uterine fibroids, or thyroid problems. If your cycle was once predictable and has shifted noticeably, or if it has never settled into a recognizable pattern past your early twenties, that’s worth investigating.
How Long You’ll Have a Cycle
Most people get their first period between ages 10 and 15. In the first couple of years, cycles are often irregular as the hormonal system matures. Cycles typically become more regular through the twenties and thirties, then start to shift again in the forties as the body approaches menopause. Most people reach menopause between ages 45 and 55, at which point periods stop permanently. From first period to last, you’ll have roughly 400 to 450 cycles over the course of your reproductive life.