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Tracking your period can give you an edge in your training and racing—and not only because it reduces the odds you’ll get caught in a porta-potty without a tampon or pad.

Exercise scientists have begun to unravel the ways in which the menstrual cycle may affect everything from your strength and power to your injury risk. The effects vary from woman to woman, making monitoring your personal proclivities even more useful.

And crucially, normal menstruation is a sign that you’re taking in enough fuel to cover all your body’s needs, which affects not only your running performance but also your long-term fertility and health, says Dr. Ellen Casey, associate attending physiatrist at the Hospital for Special Surgery.

You can now track your period alongside your workouts in Garmin Connect, or via a separate app, such as FitrWoman. But there’s an important caveat: your birth control may be manipulating your cycle and muddling its messages. Here’s what runners need to know about birth control, periods, and putting it all together.

First things first: Having a normal, regular period is a sign of a healthy balance.

It’s sometimes tempting to think of your period as a hassle, something that can complicate your race plans or stain your running shorts. But Dr. Casey encourages women to see it as a benefit.

Here’s why: all runners have a risk of developing low energy availability and a related syndrome called relative energy deficiency in sport (RED-S). These occur when the amount of energy you take in—in other words, how much you eat—isn’t enough to cover both your body’s basic functions and the energy you expend while running.

Unlike men, women who menstruate have an early warning sign they’re headed down the path toward underfueling—irregular cycles at first, and eventually, amenorrhea, or losing your period altogether. “The loss of menstruation is a reflection of the body’s efforts to prevent further energy loss,” says Dr. Liz Joy, senior medical director of wellness and nutrition at Intermountain Healthcare. “It’s saying, ‘You don’t have enough energy to protect yourself. Therefore, we’re going to turn off your ability to reproduce.’”

Low estrogen levels and insufficient calories increase your risk of stress fractures, especially when you’re doing a high-impact sport like running. In women, this is often called the female athlete triad, because it incorporates your bones, energy levels, and your period. In the long term, the female athlete triad and RED-S can also contribute to a wide range of other problems, including low immunity, depression, problems getting pregnant, and damage to your cardiovascular health.

But, your birth control might mask what’s really happening.

Hormonal contraceptives supply you with synthetic versions of reproductive hormones—either estrogen and progestin combined, or progestin only. Oral contraceptives, better known as the pill, are among the most popular, but other options include shots, implants, patches, progesterone-emitting IUDs like the Mirena, and vaginal rings.

Some of them stop ovulation, and others prevent pregnancy primarily in other ways, such as thickening the mucus in your cervix so sperm can’t get through. But they all have one thing in common: they alter the ebb and flow of your normal cycle.

Take oral contraceptives. For many types, you’ll take three weeks of hormone pills, and one week of placebo or sugar pills. During that down week, when you’re not adding hormones to your system, you’ll have what’s called a withdrawal bleed—but that isn’t exactly menstruating. “The withdrawal bleed is not the same as a normal period,” says Kirsty Elliott-Sale, Ph.D, associate professor and head of the Musculoskeletal Physiology Research Group at Nottingham Trent University.

If you found that fact surprising, you’re not alone. In fact, doctors used to prescribe the pill to athletes specifically to restore or regulate their cycles and protect their bones.

Some still do. In a new study by Dr. Casey involving more than 1,000 collegiate athletes, 65 percent said they were using hormonal contraceptives. One-fourth of those who’d had irregular menstrual cycles in the past said they were using these methods, at least in part, to regulate their menstrual cycles. Dr. Joy says she frequently sees young athletes who haven’t started or are now missing their periods—often, even high-school girls with their parents—come in requesting the pill on the advice of a pediatrician, gynecologist, or coach.

Now that scientists better understand female physiology, they know this doesn’t work. “The treatment for amenorrhea due to low energy availability is to optimize nutrition,” Dr. Casey says. Simply taking a pill that produces withdrawal bleeding doesn’t address the issue of underfueling. Furthermore, if you can’t tell whether you’re having a natural period, it’s harder to know if treatment for amenorrhea is working.

In women with the female athlete triad and low bone density, the pill may actually do further harm. Estrogen protects your skeleton—but the type of synthetic estrogen in birth control pills doesn’t have the same effect. The way it’s broken down by your liver appears to decrease levels of another key hormone, IGF-1, that’s necessary to grow new bone, Dr. Joy says. (And shots have shown an even stronger link to low bone mineral density, Dr. Casey says.)

 

We’re always taking a nutrition-first approach.

Some women with female athlete triad do benefit from an infusion of estrogen. In that situation, Dr. Joy turns to estrogen patches or vaginal rings, which are metabolized through a different pathway. But in every case, it goes hand in hand with changes to fueling. “I’m never giving somebody pharmacologic therapy for their menstrual dysfunction and low bone mineral density in isolation,” she says. “We’re always taking a nutrition-first approach.”

Hormonal contraceptives also have some other implications for runners.

Despite the large numbers of women who take them, scientists are still learning about how pills and other hormonal contraceptives influence athletic performance. As an undergraduate, Mia Schaumberg, Ph.D., coached young gymnasts who’d ask her how the pill would affect their training. “I really couldn’t find any good quality research that answered the question,” she says. “The more I looked into it, I realized that in sports science in general, there is so little research on women.”

So, she pursued her graduate degree to help reverse the trend, and is now senior lecturer in physiology at University of the Sunshine Coast in Australia. Others, too, have realized the importance of evaluating both athletes’ menstrual cycles and the impact of birth control.

It’s not going to be the difference between coming in first and coming in second.

In a systematic review and meta-analysis published last year of all the studies on hormonal contraceptives, Elliott-Sale and her colleagues found that some women who take them might have what she calls a “trivial” reduction in exercise performance, on average, compared to those who have natural menstrual cycles.

However, the amount of variation was large, and the effect minimal. “If you’re a runner, your time on the pill or off the pill is probably not going to be related directly to the pill,” Schaumberg says. “It’s not going to be the difference between coming in first and coming in second.”

Besides potential direct effects on the way your body responds to training, there are other factors to consider. Some oral contraceptives and contraceptive shots have weight gain as a side effect, something many runners would prefer to avoid. Pills also increase the risk of potentially life-threatening blood clots, Dr. Joy points out—a special concern for athletes who frequently travel.

On the plus side, the hormone-stabilizing effects of pills and other contraceptives can reduce symptoms related to your menstrual cycle—from bloating to breast pain to cramps and headaches—that might interfere with your running. Plus, they often reduce the frequency of your periods. Some people with hormonal IUDs stop bleeding altogether; a brand of pill called Seasonale is marketed with placebo pills every three months, so you have only four periods per year.

For some women with conditions like endometriosis, periods are downright miserable. “They have so much pain, and sometimes nausea and diarrhea and vomiting and migraines. It can be very disruptive in their lives,” Dr. Joy says. “For them, going three months without having to have a period is life-changing.”

Some athletes use hormonal contraceptives to manipulate when their period will come, to avoid having it during a race or trip. This practice is common, Schaumberg says, and hasn’t been shown to have any negative effects.

The contraceptive cycle was set at 21 days active and seven days of placebo pills so it would be more socially acceptable, she says, not for any physiological reason. In other words, you’re already manipulating your cycle—so it’s not that much of a stretch to tinker with it more by, for instance, skipping the placebo pills for a month or two.

The birth control method you use is a highly individual decision, so consider all these factors and talk to your doctor.

Although there are exceptions—such as relief for acne, fibroids, or migraines—most women take the pill or other hormonal contraceptives for birth control. Runners are no exception. After all, “an unplanned pregnancy will also affect performance,” Elliott-Sale says.

Discuss all your birth-control options with your doctor—preferably, one who knows about and understands your running habits and priorities. Be sure to share information about your injury history, especially if you’ve had stress fractures. All this can help you work together to find the best option for you, Dr. Casey says.

If you go the hormonal route, tracking your period won’t offer you all the same potential benefits as women who menstruate naturally. Still, you still might want to try logging your symptoms for at least three months, Elliott-Sale says. Even with muted hormonal fluctuations, you might still find cyclical trends in how you feel and perform that can affect your training.

However, know that when you’re on birth control, you can’t use your period as a gauge of whether you’re fueling properly. Instead, watch for other warning signs of low energy availability, including unexplained fatigue, drops in performance despite the same or harder training, hair loss, and a lack of concentration, Schaumberg says.

If you have a concern related to your menstrual cycle or bone health, consult a sports medicine physician or gynecologist; sports dietitians can answer questions about fueling. These professionals might test your hormone levels—women with low energy availability often have low levels of follicle-stimulating hormone, luteinizing hormone, and estradiol, Dr. Joy says—and assess your training and diet to help you get back on the right track.

“Everything is individual, we’re all different, and we make contraceptive choices for lots of different reasons. Your training and performance is one of those reasons. So it’s important to be in tune with your body and listen to your body,” Schaumberg says. And don’t be afraid to discuss these topics with your doctor, coaches, and running partners. That helps lift the taboo on talking about them for everyone. “We can support each other, and especially younger athletes, to make positive choices for their health.”