A normal menstrual cycle comes every 24 to 38 days, with 28 days being the most commonly cited average. But that 28-day number is just a midpoint. Your cycle can fall anywhere in that range and still be perfectly regular, and it will likely shift across different stages of your life.
How Cycle Length Is Counted
Your cycle length is measured from the first day of one period to the first day of the next. That first day of bleeding is Day 1. To figure out your personal pattern, track your start date for several months in a row. After three to four cycles, you’ll have a good sense of your typical range. Most people find their cycles don’t land on the exact same day every month, and that’s normal. What matters is whether the length stays reasonably consistent.
What Happens During Those Days
The cycle has two main halves, separated by ovulation. The first half, called the follicular phase, begins on Day 1 of your period and lasts roughly 13 to 14 days. During this time, your body prepares an egg for release while the uterine lining thickens. This phase is the one that varies most from person to person and cycle to cycle, which is why total cycle length differs so much.
Ovulation typically happens about two weeks before your next period starts. After the egg is released, the second half of the cycle begins. This phase lasts a more consistent 12 to 14 days in most people. If the egg isn’t fertilized, hormone levels drop and the uterine lining sheds, starting your period and resetting the clock to Day 1 again.
The period itself, the days of actual bleeding, usually lasts four to seven days. That’s just the visible portion of a much longer hormonal process happening underneath.
How Cycle Frequency Changes With Age
Your cycle doesn’t stay the same throughout your life. In the first few years after a first period, cycles tend to run longer, averaging around 30 days for people under 19. They’re also less predictable. Research from Harvard’s Apple Women’s Health Study found that cycles generally become regular within about four years of starting menstruation, as the reproductive system matures.
Through your 20s and 30s, cycles typically settle into their most consistent pattern. Then, starting in the early to mid-40s, things shift again. Cycles tend to shorten at first, averaging about 28 days for people in their late 40s, before becoming longer and increasingly irregular. After age 40, the gap between cycles can vary by 4 to 11 days on average. For people over 50, that variation jumps to about 11 days.
This late-stage irregularity is perimenopause, the transition toward menopause. It typically involves one to three years of long, unpredictable cycles before periods stop permanently, which happens around age 52 on average in the United States.
When a Cycle Counts as Irregular
A cycle is considered irregular if it consistently falls outside the 21-to-35-day window, or if the length swings dramatically from one month to the next. Specifically, if the gap between your shortest and longest cycles is more than nine days (say, 28 days one month and 37 the next), that qualifies as irregular even though both numbers individually seem fine.
Other markers of irregularity include:
- Very frequent periods: cycles shorter than 21 days apart
- Infrequent periods: cycles longer than 35 days apart, or only six to eight periods a year
- Missing periods: going 90 or more days without a period when you’re not pregnant, breastfeeding, or in menopause
- Prolonged bleeding: periods lasting longer than seven days
- Unusual flow: bleeding that’s noticeably heavier or lighter than your norm
Signs That Need Medical Attention
Some patterns in cycle frequency deserve a conversation with a gynecologist. Bleeding every week for a couple of days, bleeding in unpredictable or unusual patterns, or going three to four months between periods are all worth getting checked. If you haven’t started menstruating by age 16, haven’t developed a fairly regular monthly pattern by that age, or are older than 55 and still experiencing vaginal bleeding, those are also reasons to follow up.
Irregular cycles can stem from a wide range of causes, from stress and weight changes to thyroid issues and polycystic ovary syndrome. The pattern of irregularity, whether cycles are too frequent, too infrequent, or wildly variable, helps narrow down what’s going on. Tracking your cycle for a few months before an appointment gives your provider much more useful information to work with.