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Periods

What’s “normal”?: menstrual cycle length and variation

Top things to know:

  • The length of your cycle is the number of days between periods, counting the first day of your period until the day before your next period starts

  • For adults not using any form of hormonal contraception, a typical cycle length ranges between 24 to 38 days

  • Using hormonal birth control may change the length and variation of your cycle

The menstrual cycle is more than just your period—it’s the rhythmic changes of your reproductive system. The changes throughout the menstrual cycle are governed by hormones, which include estrogen, progesterone, follicle stimulating hormone, luteinizing hormone, and others (1). They trigger the growth of follicles (fluid filled sacs containing eggs) in the ovaries, the release of an egg (ovulation), and the growth and shedding of the uterine lining (also called the period) (1).

Your menstrual cycle can let you know when everything is working as usual, when your body is going through a change, or when something’s not as it should be. Some variation in your cycle is common, but a cycle that is consistently out-of-range may be the first noticeable symptom that something in your body isn’t quite right.

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Download Clue to track your cycle length and changes.

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What does “normal” mean anyway? 

When it comes to cycle length, there’s some very specific terminology that’s used by healthcare providers that can sound clinical and even scary in some circumstances. If you’re looking into cycle length variations, you may come across some terms like “abnormal uterine bleeding” or “irregular cycles.” 

It’s important to know that every body has variations and even though the menstrual cycle occurs in a pattern, it won’t always be exactly the same each month. Changes in this pattern are not necessarily pathological and can be considered “normal.” Healthcare providers use terms like “abnormal uterine bleeding” and “irregular” to describe variations from normal that extend beyond certain parameters (3). A variation that deviates far from the average might be normal for you with no pathological cause, while it might indicate a problem for someone else, and vice versa. The only way to know for sure is to see your healthcare provider if your cycles fall outside of “normal” to make sure there’s no root cause. 

Since the word “normal” can make people feel “abnormal,” and the meaning of “normal” varies over time and across different cultures, we choose to use other words like “typical” and “common” instead. Just know that your provider may use more clinical terms like the ones we listed above. 

What is a "typical" cycle length for people not on hormonal birth control?

Adult cycle length

A typical menstrual cycle length for an adult who is not using any form of hormonal contraception is usually between 24 to 38 days (3).

Variations in cycle lengths are common (4). Within the same year, the length between the longest cycle and the shortest cycle can vary up to 9 days and still be considered within a regular range (5). For example, a common variation can be that one cycle is 25 days long, followed by a cycle that is 33 days long.

Adolescent cycle length

The menstrual cycles of adolescents around the time of menarche (the first menstrual period) can vary greatly (6). It’s common for cycles to be variable for a few years after your first period (7). This means your periods may not always come at the same time every cycle, and they may be somewhat different cycle-to-cycle. As you progress through adolescence, cycles become more predictable to reflect adult cycle ranges, but may still be somewhat variable (6-8).

In the first few years after your periods start, a common menstrual cycle length for an adolescent is usually between 21 to 45 days, but may sometimes be longer or shorter (8).

At the onset of menarche, you may not ovulate with every cycle. As you progress through puberty, ovulation will likely happen in most of your cycles (6). 

Why cycles vary

The length of your typical cycle is determined by your age, genes, health, body mass index (BMI), behaviors, and birth control methods (9,10).

If you’ve had your period for a few years, your cycles should generally be about the same length. You may still notice changes from time to time. The length of your cycle depends on your hormones, which can fluctuate due to factors like diet, stress, jet lag, working night shifts, exercise, or taking an emergency contraception pill (the morning-after pill) (11-18). Heavy cigarette smoking, as well as chronic and excessive alcohol consumption may also affect cycle length or variation (19-21).

Menstrual cycles can fluctuate when ovulation doesn’t occur regularly. This could be due to health conditions like polycystic ovary syndrome (PCOS) or a thyroid disorder (22,23). Inconsistent ovulation and variable periods are also more likely to occur during certain phases of life, such as during adolescence, after birth, while breastfeeding, and during perimenopause (the transition to menopause) (1,24,25).

What is a common cycle length for people on hormonal birth control?

Combined hormonal birth control (e.g. the pill, the ring, the patch)

Combined hormonal birth control—like the pill, the vaginal ring, or the patch—release synthetic forms of estrogen and progesterone into your body (26). When taken as directed, the hormones in these types of birth control prevent your ovaries from preparing and releasing eggs (26). This stops your body’s usual hormonal cycling rhythm, allowing the birth control to regulate the growth and shedding of your uterine lining (1). 

The bleeding you experience on combined hormonal birth control is not a real period. It is called withdrawal bleeding because it is caused by the withdrawal of hormones during the placebo pills (pills that do not contain hormone) or during the week you don’t use your pills, patch, or ring. 

The pill

Bleeding on the birth control pill will happen during the placebo (no-hormone) days, due to the withdrawal of hormones. This will make the timing of your bleeding predictable depending on how many active (hormone-containing) pills are taken before a break.

Birth control pills are commonly dosed in packs which provide a 28-day “cycle,” with 21 to 24 days containing active hormones, and four to seven days containing pills with no hormones or not taking any pills during those days (9). Sometimes the  placebo or pill-free days happen after 84 active pills, resulting in bleeding about every three months (26). Some pills have no break in active pills at all, meaning people may have no bleeding at all while taking this type of pill (26). When you first start taking the pill or if you take your pills inconsistently, you may experience spotting or breakthrough bleeding (9,26).

The ring and the patch

The vaginal ring and patch are often dosed across a four week cycle (26). The vaginal ring contains both estrogen and progestin and is inserted into the vagina for 21 days and then removed for seven days, which then causes bleeding (26). The birth control patch is also used on a four-week schedule, with the patch changed weekly for three weeks in a row, followed by one week of “no-patch” which causes withdrawal bleeding (26). Both of these methods will cause withdrawal bleeds to occur about every 28 days (26).

Some people also decide to skip any bleeding while using birth control, by skipping over the “no-hormone” days and continuously using the active pills, patch, or ring (26). Using birth control in this way will cause you to have bleeding whenever you decide to take a break from the hormones. Bleeding will occur when you stop taking active pills, or remove the ring or patch (1). Breakthrough bleeding or spotting is common with extended and continuous use (27).

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Progestin-only birth control (e.g. the mini-pill, the shot, the implant, the hormonal IUD)

There are many different types of hormonal birth control, containing differing types and levels of hormones. Some types of birth control do not contain any estrogen and only contain progestin—a synthetic form of progesterone (28). These methods include progestin-only pills (the mini-pill), progestin injections (the shot), progestin implants, or hormonal IUDs (28).

The mini-pill

When using progestin-only birth control pills, you may not have a typical bleeding pattern. Progestin-only pills affect reproductive hormonal cycling by often preventing the ovaries from preparing and releasing mature eggs that can be fertilized by sperm (28-30). They also decrease and thin out the typical growth of the uterine lining (28).

People using the mini-pill may experience irregular bleeding, reduced bleeding, or no bleeding at all (28).

The shot and the implant

The injection and the implant both work to stop ovulation, which prevents hormone cycling (28,31). Both of these birth control methods can cause unpredictable or irregular bleeding patterns (28,31). The number of bleeding or spotting days tends to decrease over time with the implant or the shot and may result in no bleeding at all, particularly for people using the shot (32-35). All of these changes are typical with these forms of birth control.

The hormonal IUD

When using a hormonal IUD, you may not have a typical cycle. Your cycle length and period may change depending upon which hormonal IUD you have and how long you’ve had it. 

Most people will continue to ovulate while using a hormonal IUD (36-38). Ovulation is more likely the longer you've had your IUD and with the IUDs that contain lower doses of progestin (36,38,39). Bleeding pattern while using a hormonal IUD does not necessarily indicate whether ovulation occurs during a cycle—you can have no bleeding and still be regularly ovulating (36,37). 

It’s common to have irregular light bleeding or spotting, especially in the first few months after you have a hormonal IUD placed (9). Bleeding usually lessens over time and some people will stop bleeding completely while using a hormonal IUD (9).

What about the copper IUD and your cycle?

The copper IUD shouldn’t affect your cycle length since copper IUDs are non-hormonal. You will experience the same fluctuations of estrogen and progesterone across your cycle as you did when you weren’t using a copper IUD (40). That means the majority of your cycles should be 24–38 days long, which is the typical range for cycle length in adults (3). Some people using a copper IUD may notice that their period is heavier and longer, or they may experience unscheduled spotting, but these side effects often improve over time (9).

Download Clue to track your menstrual cycle length.

Article was originally published on May 17, 2018.

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