Elena Scotti/FUSION

"STRESSFUL WEEK." That's how I started my journal entry on August 9, 2010. I had just turned 25, I was not on birth control, and my period was nine days late.

Every tampon-free trip to the bathroom that week was followed by disappointment, terror, and then panic. All I could think was, "Please don't be pregnant, please don't be pregnant." The stress of potentially having an accidental baby took over every minute of my life.

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Finally, a wonderful, much braver friend of mine hauled herself to a CVS at midnight and bought me a pregnancy test, since I was too embarrassed to do it myself. It was negative.

What I didn't realize at the time is that a period can be late for a slew of reasons that have nothing to do with pregnancy, and my anxiety only exacerbated whatever was going on in my body. In fact, nearly everything—from what we eat to how we sleep to traveling abroad—can affect our menstrual cycles, which is why so many women, at some point, find themselves in the state of panic I did five years ago.

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In hopes of quelling future fears, I spoke with women's health experts about some of the most common things that can make our periods late. Here's what you should know.

Diet

Our monthly menstrual cycles are regulated by a complex system of hormones involving the brain, pituitary gland, and ovaries. These hormones interact with each other like little messengers, telling the body when to do things like ovulate and shed the uterine lining. And anything—anything—that messes with the system can mess with your period.

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One big culprit? Changes in our diet, said Jessica Shepherd, a professor of clinical obstetrics and gynecology at the University of Illinois and a practicing OBGYN. For example, suddenly deciding to go vegan—or to stop being vegan—can interrupt your monthly cycle. This is because the food we eat has a direct impact on our hormone levels.

For example, foods high on the glycemic index such as sweets, white bread, and fruit juice can increase insulin production and change the way the female sex hormone estrogen is metabolized—which in turn can affect our menstrual cycle. Another example? Not eating enough protein can affect the body's insulin and estrogen levels, which can also throw our hormones out of whack.

Stress

Stress can majorly affect our periods as well—thanks to a little guy named cortisol, also known as the "stress hormone."

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Here's how it works: When we become stressed or anxious, our bodies release both cortisol and adrenaline as part of our ancestral "fight-or-flight" response. While adrenaline makes our heart beat faster and boosts energy, cortisol works to suppress systems including our immune response, digestive system, and reproductive system.

This suppression is beneficial if you're running for your life—you want all your energy to go toward survival. But in today's world, the stress you feel is more likely coming from a boss than, say, a lion, and instead of getting released in some epic fight, it just builds up. Over time, that build-up can suppress the hormones necessary for reproduction, delaying the shedding of the uterine lining and thus delaying your period.

Big life changes

Sometimes stress accumulates gradually—and sometimes it happens all at once, which can also lead to a late period. "Traumatic events in life such as a divorce, losing a job, or a death in the family can also affect the menstrual cycle," Shepherd explained.

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The last thing you need in one of these scenarios is to let the stress of a late period add to your distress.

Sleep

Your sleep pattern can have a major impact on your menstrual cycle, since sleep deprivation—or even irregular sleep patterns—can wreak havoc on the endocrine system, and in turn, hormone function.

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This explains why something as trivial as jet lag can make your period late, said Barb Dehn, a San Francisco-based nurse practitioner who specializes in women's health. "We think it's because of the interruption of REM sleep," she told me, explaining that the exact mechanism isn't fully understood, but experts agree it comes down to hormone regulation.

Weight 

Another hormone disrupter? Changes in our weight.

Gaining too much weight, for example, can alter the body's response to leptin, a hormone that helps us feel satiated when eating. While seemingly unrelated, leptin can also affect our periods, since "a change in leptin levels can cause a change in the reproductive hormones," Shepherd explained.

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Losing too much weight can also affect our menstrual cycle. Cross-country runners, gymnasts, and professional athletes often miss periods when their body fat percentage drops below healthy levels. Similarly, extreme exercise and restrictive diets generally interfere with the body's reproductive system.

Drugs

Drugs, both prescription and street, can also delay a period—though women who use street drugs are more likely to experience these irregularities.

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"Women who recreationally use cocaine or methamphetamines may see missed or irregular periods," said Dehn. Why? Cocaine inhibits the reuptake of serotonin, norephinephrine, and dopamine—three neurotransmitters that also help regulate the "complex feedback system" that is your reproductive cycle, she explained.

Illness or an underlying medical condition

It's clear that our behavior can disrupt the delicate balance that is our reproductive cycle—but what if your behavior hasn't changed? You're sleeping fine, you're eating the same, and you haven't started taking any drugs?

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Illness is another factor to consider. If your body recently fought off a virus or infection, your menstrual cycle could be delayed—again thanks to stress, since an infection can act as a stressor on the body. However, a delayed or irregular period could also be the sign of an illness.

For example, some STDs may initially look like an irregular period. "If someone has an STD like trichomoniasis or chlamydia, they might see bleeding or spotting because the cervix is inflamed," Shepherd explained.

A missed period could also be a sign of a more serious problem such as fibroids, polyps, a thyroid disorder, or an endocrine disrupting condition such as polycystic ovary syndrome, or PCOS. While about 7% of the women in the U.S. of childbearing age are believed to have PCOS, the condition is under-diagnosed, said Dehn. PCOS can cause you to miss your period, or on the other end of the spectrum, it can cause extremely heavy and irregular periods. Other symptoms include excessive weight gain, facial hair, acne, and insulin resistance.

Birth control

And finally, there's hormonal contraception. When you first start on birth control, your periods may decrease or disappear altogether, because—you got it—anything that affects your hormones can affect your period.

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"Birth control is essentially putting your body on a regimented amount of hormones," explained Shepherd. "In the beginning, there's this push and pull, in which your body says, 'No, I'm in charge,' and the birth control pills are like, 'No, we're giving you these hormones,' and sometimes there's a struggle." Eventually the birth control will win out.

Once the hormonal birth control takes over—especially if you use a "long acting reversible contraceptives," or LARC, such as an IUD or implant—many women stop having periods altogether.

Note that, while on hormonal birth control, women are less susceptible to major changes in their menstrual cycle due to stress, illness, or the other factors we've discussed, since the body's hormones are being regulated by the contraceptive. Women using non-hormonal birth control, however—such as a copper IUD—are still highly susceptible, since the IUD does not regulate hormones.

When to see a doctor

If you miss a period one time, it's actually not that big of a deal—consider some of your recent life events to see if diet, stress, sleep, or illness might have played a role. That said, if you're sexually active and concerned you might be pregnant, you can always take a pregnancy test just to calm your nerves.

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But when pregnancy is ruled out, the experts I spoke with said that three is the "magic number"—once you've missed or had irregular periods for three months in row, you should see a healthcare provider. At that point, your doctor may test you for STDs and other conditions.

"We put a guideline on it," said Shepherd. "If someone has irregularity for three months or more, then yes, you should see your doctor." She advises patients to track their period with a period tracking app or on a calendar.

And finally, Dehn stressed that women should never be afraid to get checked out by a physician for a missed period—regardless of their sexual history, drug use, or a possible unplanned pregnancy. "We have heard it all before," said Dehn. "Our job is to take good care of you and to help make the best choices for you."

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Taryn Hillin is Fusion's love and sex writer, with a large focus on the science of relationships. She also loves dogs, Bourbon barrel-aged beers and popcorn — not necessarily in that order.