What counts as a normal menstrual cycle length
Cycle length is counted from the first day of one period to the day before the next one starts. The 28-day cycle shows up in every textbook, but very few people hit 28 days every month. Knowing the real normal range can save you a lot of worry, and it also helps you notice when something has changed.
The normal range
For adults, cycles commonly run anywhere from 21 to 35 days, according to Cleveland Clinic and the NHS. A period itself usually lasts two to seven days. Your cycles also don't need to be identical. A difference of several days from one month to the next is common and usually nothing to worry about.
Teens often have a wider range. In the first few years after periods start, cycles are often longer and less predictable because the brain and ovaries are still learning to work together, and some cycles happen without ovulation. ACOG notes that in adolescents most cycles fall between 21 and 45 days, and that it can take a few years for cycles to settle into the adult range.
Cycles also change in the years before menopause. During perimenopause, often in the 40s, they may get shorter at first and then longer and more spread out, with some skipped periods along the way.
Why the follicular phase does most of the varying
A cycle has two main halves. The follicular phase runs from the first day of your period until ovulation. The luteal phase runs from ovulation until the day before your next period.
The luteal phase is the steadier of the two. The corpus luteum, the gland that forms after ovulation and makes progesterone, has a fairly fixed lifespan. For most people the luteal phase lasts somewhere around 12 to 14 days, and it tends to stay within a day or two of that from cycle to cycle.
The follicular phase changes far more. It takes however long your body needs to grow a follicle to maturity and trigger ovulation. Stress, illness or travel can slow that process down, so ovulation happens later and the whole cycle gets longer. This is why counting forward 14 days from your period to guess ovulation often misses. A better rule of thumb is that ovulation usually happens about two weeks before your next period starts, which you only know for sure looking back.
Common reasons cycles change
Lots of ordinary things can shift your cycle length for a month or two:
- Stress, poor sleep or a big life change
- Illness, including a bad cold or flu
- Travel and time zone changes
- Large changes in weight, eating or exercise, especially intense training with too little food
- Starting, stopping or switching hormonal birth control
- Breastfeeding and the months after giving birth
- Some medications
Some medical conditions affect cycle length too. Polycystic ovary syndrome (PCOS) often causes long or unpredictable cycles. Thyroid problems, high prolactin levels, fibroids and endometriosis can also change your cycle or bleeding. These are all treatable or manageable, which is one more reason not to ignore a change that sticks around.
When to talk to a doctor
It's worth booking an appointment if any of these apply to you:
- Your periods stop for three months or longer and you aren't pregnant, breastfeeding or in menopause
- Your cycles are regularly shorter than 21 days or longer than 35 days (for teens, much longer than 45 days or more than three months without a period)
- You haven't had a first period by age 15 or 16
- You bleed for more than seven days
- Your bleeding is very heavy, for example soaking through a pad or tampon every hour or two for several hours, passing large clots, or feeling dizzy or short of breath
- You have severe pain that stops you from doing normal things
- You bleed between periods, after sex or after menopause
- Your cycle suddenly becomes very irregular after being regular
If you might be pregnant and your period is late, take a pregnancy test first. If you're trying to conceive and your cycles are irregular, mention it early, since irregular cycles can mean ovulation isn't happening every month.
Getting to know your own number
The most helpful number is your own average, and that takes a few months of tracking to find. Log the first day of each period and you'll soon see your usual length and how much it moves around. That record is also useful at a doctor's appointment, where "my cycles run 26 to 31 days" is much easier to work with than "they're kind of all over the place." Barkat's cycle tab keeps this log private on your phone and works out your averages for you.
This article is general information and not medical advice.
Sources
- Cleveland Clinic: Menstrual Cycle (Normal Menstruation)
- NHS: Periods
- ACOG Committee Opinion 651: Menstruation in Girls and Adolescents: Using the Menstrual Cycle as a Vital Sign
- Mayo Clinic: Menstrual cycle: What's normal, what's not
This is general information, not medical advice. If something about your cycle worries you, talk to a doctor or midwife.