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Typically, we understand Aunt Flo to be nature’s memo that you aren’t pregnant this month. A new article series at Natural Womanhood (disclosure: Verily’s Associate Editor Mary Rose Somarriba is the editor of Natural Womanhood), titled Reasons Women Need Periods, shares that periods, or more precisely, our fertility cycles that result in periods, are multi-tasking health warriors. The hormones in our cycles battle to protect us against a wide range of women’s foes, from HPV infection to cardiovascular disease. Yes, we do need our periods—our bones, our hearts, even our immune systems are counting on it!

Her lovely bones

Women have amazing skeletons under our skin. They’re beautiful, but vulnerable. As we get older our bones are at higher risk for breaking, which can mean a reduced quality of life and even threaten our lives.

Natural Womanhood’s Cassie Moriarty investigated the role that our cycle hormones play in women's bone health. “For years, researchers have surmised that post-menopausal bone loss is attributed to the lack of estrogen that occurs after menopause. But estrogen isn’t the only thing post-menopausal women lack,” she writes. “After cycles cease, progesterone is low as well. Mounting research indicates that bones need progesterone just as much as estrogen.”

Researcher Dr. Jerilynn Prior, who studies hormones and bone health, told Moriarty that the progesterone in a girl’s cycles when she’s young can have a direct impact on how much bone mass she builds up in the first decades of her life. How often, and how healthfully, she ovulates helps determine how much bone she has to work with after she reaches old age.

“In other words, if you’re not ovulating during your prime reproductive years, your bone density will be negatively impacted, and those effects will manifest in your post-menopausal years,” Moriarty writes.

So getting your period when you’re young is essentially like putting bone in the bank. 

The heart of the matter

Heart disease is a global epidemic, and the number one killer of women in the United States. So why don’t more women know about how hormones can affect our cardiovascular health?

It turns out that both estrogen and progesterone affect the human heart. “We also know that estrogen has a preventative effect on coronary artery calcification (CAC). CAC can cause blood vessels to narrow and lead to the development of heart disease, but estrogen helps prevent the buildup of calcium in the arteries,” Moriarty writes. “Estrogen also positively influences myocardial contractile reserve, which gives the heart the ability to pump harder when it needs to, and to rest easier when it doesn’t.”

As for progesterone, it plays a role in decreasing blood pressure. Even a woman’s resting heart rate is affected by her cycle. During the first, estrogen-heavy half of her cycle, her heart beats slower; during the second half, when progesterone takes over, her heart pumps faster.

Grant her immunity

In the age of COVID-19, the human immune system and its many mysteries are top of mind. More than ever, it’s clear that an immune response that reacts well enough—but doesn’t overreact—to viral and bacterial intruders can be the difference between life and death. And yes, our menstrual cycles play a role here too.

As Moriarty explains for Natural Womanhood:

In the follicular phase (the first half of the cycle) estrogen is dominant. Since estrogenic cervical fluid “opens up” the reproductive tract to sperm (and therefore other foreign microbes) during this phase, the immune system needs to be on high alert to filter out potential invaders.

In the luteal phase (the post-ovulatory phase of the cycle), progesterone is dominant. During this phase, gestagenic cervical fluid seals the cervix up (as well as the rest of the reproductive tract) and immunity is temporarily suppressed; this allows the body to accommodate a potential pregnancy. Pregnancy, after all, is an immunity conundrum; in any other scenario, the body would attack the arrival of a separate entity. Thanks to progesterone, the body has a workaround so that the immune system gives the baby a hall pass to stick around inside mom’s body for a bit.

Crucially, the fertility cycle also helps a woman’s cervix develop. As she is exposed over time to rising and falling hormones, her cervix matures at a healthy rate, with a natural ability to fight infections like HPV, the virus that causes almost all cases of cervical cancer.

Does hormonal birth control offer the same benefits?

What if you’re on the Pill or a hormonal IUD? Are your bones, your heart, and your immune system reaping health benefits from constant exposure to these amazing hormones?

Unfortunately, the hormones in artificial contraception aren’t the real deal. They’re synthetic. Instead of natural progesterone, the Pill gives women a drug called progestin. Instead of estrogen, they get ethinyl estradiol, also known as EE. These hormones, at best, only mimic what our bodies do naturally—and they just don’t have the same health effects.

What many women don’t know is that medications like the Pill actually shut your fertility cycle down. That’s how those drugs prevent pregnancy (except of course, when they don’t). Although many forms of birth control are specifically marketed as causing “no periods” or “few periods,” the reality is that all forms of hormonal contraception are, in fact, “no period.” The bleeding most women experience at the end of every month on the Pill is actually her body going into withdrawal from the artificial hormone progestin, not part of a real cycle. It looks like a period, but it’s a fake-out.

The cycle-suppressing effects of hormonal contraception mean a woman’s body doesn’t experience the natural dance between estrogen and progesterone, with estrogen “zigging” to progesterone’s “zagging,” as Moriarty puts it. No ovulation means no real period—and no benefits from the fertility cycle.

In fact, the Pill is actually associated with risks to a woman’s skeleton, cardiovascular system, and immune system. It can cause deadly blood clots and debilitating strokes, which is why doctors warn women over 35 and smokers of all ages to stay away from the meds—those groups are already at higher danger for cardiovascular problems, making the Pill’s risks even worse. High blood pressure and type 2 diabetes are also linked to hormonal contraception.

Research has tied the Pill to a negative effect on bone density, too. And what about our immune systems? Oral contraceptives are linked to accelerated cervical maturation. The more rapid pace of development could leave women and girls more vulnerable to infection with diseases like HPV. Good things, like natural immunity, take time.

Our culture has trained many women to feel like their periods are at best, a necessary evil, and at worst, a “curse” to be avoided if at all possible. But as Moriarty says, “The more we learn about feminine health, the more reasons we encounter why #womenneedperiods.” The fertility cycle is not a foe, but an often-misunderstood friend. Check out the entire article series at Natural Womanhood to learn more about your monthly reminder that your cycle is fighting for your healthy future.