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Can traveling really make your period late?

Inquiring uteri want to know.

When it comes to travel, sometimes the only thing worse than a delayed plane is a delayed period that you were expecting. On a recent work trip, my period—which is typically on a pretty regular schedule—didn’t arrive when it should have, leaving me in the lurch. Would it appear in the middle of a long car ride? During sunrise yoga? On the 11-hour flight home? Wait—was I pregnant?

In the end, my period showed up four days after its anticipated arrival. I started to wonder if my traveling was behind its belated appearance. As it turns out, there is something to this theory. Before you spend time on your next trip trying to find a pregnancy test in a foreign pharmacy, here’s what you should know about how travel can affect menstruation.

For starters, traveling can disrupt your circadian rhythms, which play an important role in how your hormones work.

Circadian rhythms are one main reason travel seems to affect periods, Carla Bossano, M.D., assistant professor of gynecology and obstetrics at the Johns Hopkins University School of Medicine, tells SELF.

In the simplest terms, your circadian rhythms are the ways your body changes in response to intervals of light and dark. Among many other processes, your circadian rhythms help to regulate your body’s hormonal fluctuations, according to the National institute of General Medical Sciences. As you probably know, your hormones dictate a lot about menstruation.

You get your period when, as a result of an egg you released during ovulation going unfertilized, your levels of estrogen and progesterone drop, prompting your uterine lining to slough off. Doing something that messes with your circadian rhythms could possibly change this process and have a ripple effect on your period.

For instance, if you’re traveling across time zones, your circadian rhythms can get all out of whack so that your body is still running on the time from the place where you departed, the National Institute of General Medical Sciences explains. Voila: Now you have jet lag. Just as your sleep cycle can suddenly be off kilter when you’re jet lagged, so can your usual hormonal timetable.

Traveling can also lead to you sleeping less, whether because you’re trying to power through the aforementioned jet lag or because you spend nights out exploring a new city. “Lack of sleep messes with those normal rhythms and those normal hormone secretions,” Dr. Bossano says, which in turn can change when you menstruate.

If it’s not your circadian rhythms that influence your period during travel, it could be the stress.

“Any sort of stress, even ‘good’ stress like a vacation, can throw off your cycle,” Dr. Bossano explains. According to the Mayo Clinic, stress affects your brain’s hypothalamus, which is the control center for the hormones that govern your menstrual cycle. Stress can alter these hormones enough that it causes a delay in ovulation or makes you skip ovulation altogether, Dr. Bossano says. That can (sometimes but not always) stave off your period’s arrival.

If you find you get peak stressed when you travel, try these tips to make the whole thing go as smoothly as possible.

Travel could also make you sick, which might in turn hold up your period.

According to the Cleveland Clinic, illness is one of many things that can make your period act abnormally. When you’re sick, your body devotes a lot of resources to handling that threat to your system, so processes like menstruation can fall by the wayside, Sarah W. Prager, M.D., associate professor in obstetrics and gynecology at the University of Washington, tells SELF.

Unfortunately, getting sick is sometimes just part of the deal when you’re traveling. There’s the simple fact that a lot of people touch surfaces in places like airplanes and airport bathrooms, so you can be exposed to a lot of germs that may spread illness. Also, travel often necessitates being sardine-style packed in with tons of other people, and some of them could be sick, possibly passing their illnesses on to you through the air. Finally, if you’re going somewhere with cuisine you’re not used to, or you accidentally chug some water that’s not safe for drinking, you might end up with a case of traveler’s diarrhea that throws your body for a loop.

It’s worth noting that if you’re on hormonal birth control, you probably won’t be as susceptible to these hormonal fluctuations.

That’s because those hormones are typically powerful enough to override the influences that travel, stress, and illness could otherwise have on your period, Dr. Prager says. So, if you’re taking hormonal birth control and your period suddenly goes missing, you can pretty quickly assume it’s not travel-related.

That said, your birth control could be behind a seemingly delayed period. Some types of contraception, like certain birth control pills and hormonal IUDs, can cause people to stop having periods completely, Dr. Prager says. They typically do this through progestin, a synthetic version of the hormone progesterone, which thins your uterine lining to the point where there may not be enough to shed in a period. If you happen to be traveling when that side effect goes into action, it may seem like your period is late when, in reality, your body’s adjusting to your birth control’s power over your uterine lining. Or, if you went on a trip and forgot your birth control (or messed it up a bit while you were out of town) it’s possible that could throw your period for a loop. If that’s the case, make sure you’re using a backup method of birth control if you’re concerned about pregnancy.

Always keep in mind that a late period could be a sign of pregnancy that happens to coincide with your trip.

If you think there’s a real chance you could be pregnant, there’s no harm in taking a pregnancy test, Dr. Prager explains, especially if you’re experiencing other signs of early pregnancy such as breast soreness, nausea, or vomiting. Urine pregnancy tests are often highly sensitive now, Dr. Bossano says, and are typically reliable by the time your period is late. (Though they become more accurate the longer you wait, since your body starts to produce more pregnancy hormones as time passes.)

Even though these are all potential reasons your period can show up late when you travel, beyond pregnancy, it’s usually hard to pinpoint the exact cause of a delayed period.

If your period only goes MIA once, it’s likely no cause for concern. However, if you miss three or more periods—and you’ve got a negative pregnancy test—the Mayo Clinic suggests you talk to your doctor to see if you can find an underlying cause, then figure out how to fix it.

Written by Colleen Stinchcombe.

This article originally appeared on Self US.

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