Unlike your internet bill (which arrives like clockwork every month) your period isn’t on a neat 30-day schedule. “A ‘regular’ period is one that comes every 21 to 35 days,” explains Kate White, MD, associate professor of ob-gyn at Boston University School of Medicine. In fact, it’s pretty uncommon for a person to bleed every 28 days like clockwork, says White. “What’s more common is having cycles that last somewhere between 25 and 32 days, and cycles that are a little bit different in length each month,” she says.
But what if your period isn’t showing every 21 to 35 days? Well, it turns out there are quite a few factors that can cause your menstrual cycle to go haywire (or even pull a disappearing act altogether). Here are 10 common reasons why your monthly flow isn’t showing up when it should.
- You’re stressed.
All that pressure you’re under at work? It could be to blame for your out-of-whack period, as stress can interfere with the communication between your brain and ovaries, says White. “The most common result is a skipped period (or two),” she says. Soma Mandal, MD, New York City-based internist and women’s health specialist, notes that stress can raise cortisol and insulin levels, which can interrupt your menstrual cycle. While it’s not dangerous to miss a period or two once in a while, once you’ve missed three in a row, it’s time to see your gyno, adds White.
- You have an IUD.
While IUDs are all sorts of amazing at preventing pregnancy, they can leave you with an irregular period. “Intrauterine devices, especially the kind coated in hormone, decrease blood loss and are used to treat heavy bleeding,” says Nisha Jayani, MD, endocrinologist with Paloma Health. “Therefore, they can cause a lack of period or spotting.” So if you have a hormonal IUD, keep in mind that you might experience very light periods, bleeding only every several months, or no regular bleeding at all, says White.
The copper IUD, which is hormone-free, will have a different effect from the hormonal varieties: You may actually experience a heavier flow and more intense cramps at the start, which should improve after three or four months, says White. “All IUDs can cause spotting in-between your periods for the first three to six months as your body adjusts to the device,” she adds.
- You’re pregnant.
The most obvious reason your period is MIA: pregnancy, of course. “When you become pregnant, your body retains the lining of the uterus to nourish and create a safe environment for the fertilized egg,” says Mandal. “This is why your menstrual cycle stops during pregnancy.” If you miss a period and notice other signs of pregnancy, like tender, swollen breasts, nausea, fatigue, or an increased need to pee, take a home pregnancy test or visit your doc.
Oh, and BTW: Once you’ve given birth, don’t expect your cycle to go back to normal right away. “Your periods may take a few months to settle back into their usual routine,” says White.
- You have PCOS.
People with polycystic ovarian syndrome (PCOS) have hormonal irregularities which can cause a messed-up monthly flow. “There is usually an excess of testosterone and insulin, which can cause menstrual irregularities,” says Mandal.
One of the irregularities you might experience with PCOS: unpredictable timing. “Your period may come months apart and then may come twice in a month,” says White. Your flow could be unpredictable period to period too: “The flow can range from light spotting to quite heavy flow,” says White. “Those periods that come after months of no bleeding in a row can be very intense.
- You recently switched birth control methods.
Didn’t love your last BC? Swapping it out for something different could throw your period out of whack for a while. “Any time you change hormones, whether it’s changing your pill formulation or changing one form to another—like switching from the pill to the ring or from the patch to the injection—it can take some time for your body to adjust,” says White.
The most common change you’ll notice when you first go on a new method of birth control is vaginal spotting for a few weeks or months. “Spotting doesn’t mean the method isn’t working, it just means your body is getting used to the new hormone cocktail,” says White.
- You have a thyroid issue.
Hyperthyroidism (when your body produces too much thyroid hormone) and hypothyroidism (when your body produces too little thyroid hormone) can both have an effect on your menstrual cycle if left untreated. “An untreated underactive thyroid can cause very infrequent or irregular periods,” says Jayani. “An untreated overactive thyroid can cause periods to cause more or less frequently and eventually lead to stopping of periods.”
White says one of the first things a gyno will do if you have an irregular, heavy, or absent period is check your thyroid hormone levels through a blood test. If it turns out you’ve got hyperthyroidism or hypothyroidism, the good news is, once you get the condition under control, your period should return to normal, says Jayani.
- You’re breastfeeding.
“Breastfeeding can suppress periods for six months or more, especially if you’re exclusively breastfeeding,” says White. Here’s why: When you’re breastfeeding, you have higher levels of prolactin, the hormone that causes you to produce milk, says Mandal. “This hormone also stops ovulation and can therefore cause you to have no menstrual cycle,” she explains. That said, it’s still possible to get pregnant while breastfeeding, says Jayani, so you’ll want to be diligent about using contraceptives.
- You over-exercise.
Exercising too much can cause your period to go into hiding. This is especially the case for gymnasts, ballet dancers, and some runners, says Jayani. The lack of a period due to exercise is called “athletic amenorrhea.” It typically occurs in those who are very lean. Intense exercise and extreme thinness can cause a drop in estrogen and progesterone, the hormones that regulate your monthly cycle.
Treatments for athletic amenorrhea include changing up your diet so that you get enough calories to power through your workouts, upping your calcium intake, using birth control to get your hormones back into balance, and exercising less.
- You have uterine fibroids.
Uterine fibroids and uterine or cervical polyps are benign lesions in your uterus or cervix that can cause spotting in between periods or heavier flow, according to White. “For some people with large or many fibroids, their periods can be so heavy that they need a blood transfusion,” she says. “But fibroids and polyps won’t change the timing of your periods.”
You may also experience pelvic pain and pressure in the lower abdomen with fibroids, notes Jayani. While there’s not one single best way to treat fibroids, some medications and non-invasive procedures exist. If you think you have fibroids or polyps, head to your doc. They can diagnose polyps through a routine pelvic exam; to diagnose fibroids, you’ll need to have an ultrasound or lab tests done.
- You’re beginning menopause.When you reach this phase of life, your body starts preparing to cease the reproductive cycle. “This causes a decrease in hormones such as estrogen and progesterone,” says Mandal. “When this occurs, women can experience an irregular cycle, heavier or lighter cycles, or missed periods.” Eventually, you’ll stop getting your period altogether.
The bottom line: There are so many factors that can lead to an irregular cycle, and what’s normal for one person won’t necessarily be normal for someone else. Visit your doctor if something seems off about your period so you can get your monthly flow back to what’s normal for you.