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It’s often referred to as your “monthly” period. But hormonal birth control users aside, there aren’t actually a whole lot of people out there on an exact 28-day cycle. So what counts as the average time between periods, and what’s maybe outside the realm of “normal”?

Your menstrual cycle is your body’s way of getting ready for a baby — over, and over, and over. Each month your body preps your uterus for pregnancy by lining the uterus with blood and tissue, while your ovaries release an egg that’s ready and waiting for sperm.

If the egg doesn’t get fertilized, the uterine lining is shed in the form of your monthly period. Rinse and repeat until you become pregnant, go through menopause, or if you go through a procedure that changes the reproductive system.

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Despite the straightforward process, there’s always room for some variation. Some menstrual cycles are naturally longer or shorter than others — meaning periods are further or closer together. What’s more, other stuff that’s going on with your body can sometimes affect your cycle length too.

So let’s take a look at what’s typical to see where you stand, the factors that can contribute to irregular periods, plus what you can do to get things more in sync.

An average menstrual cycle is 28 days long, meaning there are 28 days between the start of one period and the start of the next period.

But average doesn’t necessarily mean typical, or even normal. Only 10 to 15 percent of women have 28-day cycles, and medically speaking, a normal menstrual cycle can be anywhere from 21 to 35 days long.

Another thing: It’s also completely OK for your cycle to be a little longer or a little shorter from month to month. While some people can predict the exact day (and maybe even the exact time of day) that their period will come, that’s definitely not the case for most.

Menstrual cycles are considered longer than normal if you go more than 35 days between periods. Called oligomenorrhea, it can have a number of different causes:

  • Your age. Cycles tend to be a little longer in the first few years after getting your period.
  • Your weight. Having too little body fat can cause your periods to slow or even stop. That can happen as the result of an eating disorder or extreme exercise or athletic training. Being obese can have an impact too.
  • Certain ovarian conditions. Polycystic ovarian syndrome (PCOS) and ovarian insufficiency syndrome (aka early menopause) can both slow or stop your periods.
  • Thyroid problems. Specifically, an underactive thyroid can slow the production of hormones that trigger ovulation.
  • Mental health issues. Both depression and high rates of perceived stress are associated with irregular cycles. It’s allll connected, people.
  • Pituitary tumors. Your pituitary gland is responsible for making the hormones that control your menstrual cycle. But pituitary gland tumors (which are usually benign) can throw that system out of whack.
  • Menopause, early or right on time. The transition to menopause involves some wild hormone swings, which means your bleeding can become irregular or even disappear altogether for a month or 2.

Longer-than average cycles aren’t automatically problematic (especially if it’s a one-off thing), but they can often be a sign of an underlying issue. People who have fewer periods (e.g. their period occurs just a few times a year with several months in between) have a higher risk of developing endometrial cancer.

What’s more, long cycles could make it harder to get pregnant, since they could indicate that your body isn’t ovulating as it should be. So if you notice your periods are perpetually stretched out, it’s worth talking to your doc.

Very short cycles can make it seem like you’re getting two periods in a month, which is so, so fun. But what you’re probably dealing with is one period plus spotting, which can have a bunch of possible causes. Behold:

  • All things pregnancy-related. You might experience implantation bleeding in the very early days of being pregnant, before you even know there’s a bun in the oven. Miscarriage — which can also often happen before you have any idea you’re pregnant — can also cause spotting.
  • Hormonal issues. Namely PCOS, which can cause bleeding between periods.
  • Fibroids and cysts. Both types of growths are common, and usually harmless. But they’re notorious for causing heavy bleeding and spotting that might seem like a bonus period.
  • Endometriosis. In addition to having normal period bleeding, women with endometriosis also have to deal with bleeding from cells and tissues outside of the uterus. And it can be heavy, irregular, and painful.
  • STIs. Chlamydia and gonorrhea can both trigger heavy bleeding and bleeding in between periods. Talk about the gifts that keep on giving.
  • Menopause, early or right on time. Again, HORMONES. Just like they can make your cycles longer, they can make them shorter or cause spotting.
  • Polyps. Either in the uterus or on the cervix, these can also cause more bleeding.

Just like with too-long cycles, ones that are too short or super spotty can definitely be a sign of an underlying problem. And if you’re trying to get pregnant, the issue might have an impact on your fertility. So they’re worth bringing up with the doc if they start to happen frequently.

Keeping tabs on your period is definitely a good idea. Yeah, you’re less likely to have to make an emergency run for tampons, but that’s not all. Since your cycle can clue you in to other things that might be going on in your body, it’s sort of a window into your overall health.

And if you’re trying to get pregnant, knowing when your periods are happening makes it easier to figure out when you’re ovulating.

And it’s pretty easy. You can go the old school route and just mark when your periods start and stop on a calendar. After a few months you’ll have a pretty good sense of how long your cycles are and when your next period is likely to occur.

Apps are another option, and they can be kind of awesome.

Ones like Ovia and Glow use your data to predict when your next period will come, basically doing the work for you. They’ll also tell you when you’re likely to be ovulating so you know the best time to have sex if you’re trying to get pregnant (or definitely avoid it).

If your periods feel straight up out of control, simple lifestyle changes might be enough to rein them in. You can try things like:

  • Get to a healthy weight. Being both under- or overweight can both affect your cycles. If you’re struggling with an eating disorder, seek medical attention for help on how to achieve a higher weight in a healthy way.
  • Manage your stress. Find some stress relievers that feel doable for you and try incorporating them into your days. As your mood improves, your cycles might start to even out.
  • Work it out. Regular exercise helps keep your weight in check, and some research suggests that it might improve period regularity. Yoga seems to be particularly beneficial, but it might take several months of regular practice to reap the full benefits.
  • Rule out underlying conditions. If your period chaos is caused by a health problem, addressing the root cause could get things back on track.
  • Consider birth control. The pill makes your periods come ’round like clockwork, and has the added bonus of making them lighter too.

Occasional cycle strangeness is actually pretty, well, normal. But you should check in with your doctor if your periods are fewer than 21 days apart or more than 35 days apart.

In some cases it might just be a weird month, but other times you could have an underlying health issue that needs to be addressed.

Other period weirdness that warrants a call to the doctor:

  • Your periods start getting erratic or painful, seemingly out of nowhere.
  • You haven’t had a period in more than 90 days and you aren’t pregnant.
  • You’re experiencing spotting.
  • Your periods go longer than 7 days or are super heavy.
  • There’s a big difference between your longest and shortest cycle.
  • Your periods are irregular and you’re trying to get pregnant.

tl;dr

Most menstrual cycles aren’t exactly 28 days long. Really, anything between 21 and 35 days falls within the realm of OK, and it’s normal to experience some variation from month to month. But you should let your doctor know if your periods are longer or shorter than that, since highly irregular cycles could mean something’s up health-wise and can also affect your ability to get pregnant.