A typical menstrual cycle lasts 21 to 35 days in adults, counted from the first day of one period to the first day of the next. The often-cited “28-day cycle” is just an average, not a standard you need to hit. Your cycle can fall anywhere in that 14-day window and still be perfectly normal. The bleeding portion of the cycle, your actual period, typically lasts 2 to 7 days.
What Counts as Day 1
Your cycle starts on the first day of full menstrual bleeding, not spotting. It ends the day before your next period begins. So if you start bleeding on March 3 and your next period arrives on March 31, that cycle was 28 days long. Tracking a few cycles gives you a much clearer picture of your personal pattern than relying on a single month.
The Two Main Phases of Your Cycle
Your cycle splits into two key halves, separated by ovulation. The first half, called the follicular phase, runs from the start of your period until you ovulate. This phase ranges from 14 to 21 days and is the main reason cycle lengths vary from person to person and month to month. During this time, your body is preparing an egg for release, and how quickly that process wraps up determines when ovulation happens.
The second half, the luteal phase, runs from ovulation until your next period begins. This phase is far more consistent, lasting about 14 days regardless of your overall cycle length. If your cycle runs long one month, it’s almost always because the first half took longer than usual, not because the second half stretched out. This is important to know if you’re tracking ovulation for fertility purposes: a longer cycle usually means you ovulated later, not that something went wrong after ovulation.
How Cycle Length Changes With Age
Your cycle doesn’t stay the same throughout your life. In the first year or two after a first period, cycles tend to run longer, averaging about 32 days. This is because the hormonal signaling system is still maturing, and ovulation doesn’t happen every month yet. About 90% of cycles during adolescence fall between 21 and 45 days, and occasional cycles shorter than 20 days or longer than 45 days can happen without indicating a problem. By the third year of menstruating, 60 to 80% of cycles settle into the adult range of 21 to 34 days.
Cycles tend to be most regular during your 20s and early 30s. Then, as you approach menopause (a transition called perimenopause, which can start in your 40s), things shift again. Ovulation becomes less predictable, and cycles may get shorter, longer, or both. An early sign of perimenopause is a consistent change of seven days or more in your cycle length. Later in the transition, gaps of 60 days or more between periods are common before menstruation stops entirely.
Why Your Cycle Might Be Irregular
Occasional variation is normal. A cycle that’s 26 days one month and 30 the next doesn’t signal a problem. But consistently irregular cycles, or ones that regularly fall outside the 21-to-35-day range, can point to something worth investigating.
Polycystic ovary syndrome (PCOS) is one of the most common causes of long or unpredictable cycles. Higher-than-typical levels of androgens (a group of hormones) can prevent the ovaries from releasing an egg, which delays or skips periods entirely. Cycles longer than 40 days between periods are a hallmark pattern. PCOS is typically diagnosed when someone has at least two of three features: irregular periods, elevated androgens, or characteristic changes on an ovarian ultrasound.
Thyroid problems also directly affect cycle length. Both an overactive and underactive thyroid can make periods lighter, heavier, or irregular by disrupting the hormonal chain reaction that triggers ovulation. An underactive thyroid can also raise prolactin, a hormone normally involved in breastmilk production, which can suppress ovulation on its own.
How Stress Disrupts Your Cycle
Stress is one of the most common and least recognized reasons for a late period. When your body is under stress, whether physical or emotional, it ramps up cortisol production. Higher cortisol suppresses the reproductive hormone signals that trigger ovulation. Without ovulation happening on schedule, progesterone (the hormone that sets up and times your period) drops, and your cycle either lengthens or skips entirely.
This isn’t a malfunction. Your body is essentially deprioritizing reproduction when it senses that conditions aren’t safe, even if the “danger” is a work deadline or a difficult life event. A single stressful month can push ovulation back by a week or more, which makes the whole cycle longer without anything being structurally wrong. Once the stress resolves, cycles usually return to their normal pattern within a month or two.
How Much Bleeding Is Normal
During your period, you can expect to lose about two to three tablespoons of blood total, spread across 2 to 7 days. That’s less than it often looks. Bleeding that soaks through more than one pad or tampon every hour or two is considered heavy and worth looking into. Most people find that flow is heaviest in the first two or three days, then tapers off.
When a Missing Period Needs Attention
If you’ve been menstruating and your period stops for three months or more without an obvious explanation like pregnancy, that meets the clinical definition of secondary amenorrhea. For teens, not having a first period by age 15 is classified as primary amenorrhea. In either case, the absence usually points to a hormonal imbalance, a thyroid issue, extreme weight change, or another condition that’s interrupting the ovulation process. Three months is the threshold where evaluation becomes important, because the underlying cause often affects more than just your period.