Penny Lane looks like a fertility goddess reincarnated as a Sports Illustrated swimsuit model. But she hasn’t always looked so amazing. When she was younger, she forced her body to fit the restrictive standards of the modeling industry. She worked out and dieted so much that she lost her period, a telltale sign of dangerously low weight. She knew something was wrong. So she met with a holistic doctor who told her to cut the cardio and eat foods that would restore her reproductive hormones.

It worked—dramatically. “I went from a B cup to a double G cup within a space of like a few months,” she says. For those who don’t speak bra size, this is large enough that she deserves her bragging rights. Her once-bony hips have filled out. Her thighs are no longer sticklike but round and toned and strong. Her waist could be called wasplike if a wasp could have rippling abs. She is no longer cute. She is seriously hot.

“Have you done anything to your body?” people ask her. Translation: Has she gotten breast implants? Taken some kind of steroid that targets the glutes and mammary glands? “The answer is no,” she responds. “My body is completely natural. It’s not surgery, and it’s everything to do with hormones.” Now a certified holistic health coach who promotes “healthy body standards” in bikini competitions, Lane models and produces workout videos for the app 28 Wellness, a menstrual cycle tracker that claims to perform the kind of hormonal magic that turned her into the physical wonder that she is today.

More specifically, 28 Wellness promises to regulate the hormones tied to a woman’s cycle and mitigate common menstrual symptoms—cramps, bloating, migraines, and the rest—through nutrition and exercise. “Move, eat, and live in sync with your cycle to balance your hormones,” its ad copy reads. It’s not the first app to make this claim, but it is the prettiest. 28 Wellness was created in 2020 by Brittany Hugoboom and her husband and received seed funding from Thiel Capital. Like the Hugobooms’ other project, Evie magazine, the app presents a vision of the world where the ideal woman is a busty princess or a busty supermodel or a busty milkmaid. To realize that vision, women must embrace their “feminine energy,” which means eating and working out in accordance with its daily prescriptions.

The app is part of the Evie-verse’s resistance to what Hugoboom calls “girlboss feminism,” in which the women who have felt shamed because they “choose conventional roles” over “casual sex, careerism or ideological activism” finally get the life that they really want. “I think more women want a soft life, a beautiful life, than feeling all this pressure to do all these things,” Hugoboom recently said. Feminism is too much pressure, too many masculine expectations imposed from the outside; Evie is feminine, a “conservative Cosmo.” Its website features seasonal fashion tips, listicles about non-toxic perfumes, how-tos on dressing like your “female archetype,” opinion pieces from women who want their pre-pregnancy body back, and a cover story featuring a busty model smoldering at the camera in a string of pearls and a lowcut black silk-and-lace version of Evie’s “raw milkmaid dress.” Its title: “The New Sexual Revolution.”

Sex is very much a part of Evie’s brand. The magazine recently announced its “Sex Issue” at an “EROS party” thrown in Manhattan. “Progressives own sex positivity but abandoned marriage and monogamy a long time ago,” an Evie reader had written to the magazine. “Conservatives own marriage but can’t bring themselves to be sex positive.” Hugoboom responded, “Your sex life with your husband is arguably the most important part of your marriage.” Then she set about to show her readers how to do it. The new issue promises to be the “most complete, thoughtful, and beautiful guide on sex and intimacy ever put into print.” It’s a steamy affair, and as the action gets more intimate, photography gives way to “beautiful hand-drawn illustrations.” The New Sexual Revolution is “sex positive,” but unlike the reckless feminism of the past, it draws the line at “decoupling sex from commitment.” It is the experience of liberation, but safely in the home, or at least safely filtered for Instagram.

The 28 Wellness app is where this aesthetic hits concrete—or a beachside yoga mat. It features “supermodel trainers” who help women “sync their cycle” and release their “feminine energy.” 28 Wellness promises to turn users into the glorious nymphs of a Rubens painting while allowing them to maintain a level of control over their fertility at least equal to that currently enjoyed by women on the pill. To achieve this, it relies upon “hyperpersonalized” algorithms. But these algorithms fail to provide the kinds of results obtained by true individual observation; for this reason the app—and others like it in the wild world of femtech—isn’t actually so great at tracking fertility. And this is a serious problem for 28 Wellness’s new brand of feminism. By conflating health with sex appeal, it elides the real complexities of reproductive health that women need to understand to manage their cycles well.

The first thing I notice when opening the app is that no one is wearing a shirt. There are bikinied supermodels everywhere, loping through forests, stretching on beaches, and sluicing through water of the purest blue. These goddesses are my guides through the four phases of the menstrual cycle: menstrual, follicular, ovulatory, and luteal.

When prompted I enter my cycle length as “irregular” (it hasn’t yet returned since the birth of my youngest daughter). The app assigns me to the luteal phase, which I am told is the time to “Balance” as I prepare for menstruation. A lipsticked model in an off-shoulder tulle dress encourages me to treat myself with “extra kindness” today. That’s because, according to the daily update on my body, featuring a model in white lace lingerie, my “feel-good hormones are on the decline” and my P.M.S. symptoms may be starting. The nutrition section is paywalled—28 Wellness was once free but now charges a one-hundred-seventy-nine-dollar annual fee—but seems to consist largely of smoothie recipes and variations on sweet potatoes (root vegetables and tubers are recommended for preparing my body for my period).

In the workout section, a model in a fuchsia bikini leans provocatively on a tree branch and shows me her “Ballerina Thighs,” which I can have too if I follow her nineteen-minute workout (also paywalled). This feature, according to Google Play Store reviews, is one of its main value-adds. A number of other femtech apps give general workout tips—chill on the cardio during your period, basically—but 28 Wellness offers a full suite of exercises led by its supermodel trainers.

This connection between inspiration and aspiration—not just alleviating symptoms or promoting fertility awareness but improving your image—is “what 28 is all about.” The description on Lane’s promotional video informs me that “your happiest, healthiest, and sexiest self comes from balancing your hormones (and your cycle holds the key).”

But do balanced hormones make you sexier? And can 28 Wellness, with its aspirational eating and exercise tips, deliver on its promise to alleviate the symptoms of menstruation, including how it influences our perceived attractiveness?

This turns out to be a complicated biological question. I know from my self-taught sex ed in the school of Google that a bewildering number of factors can affect the hormones involved in a normal menstrual cycle: estrogen for building a new uterine lining; L.H. or luteinizing hormone to stimulate the release of an egg, F.S.H. or follicle-stimulating hormone to tell that egg to mature, progesterone to signal the end of the cycle if the egg hasn’t been fertilized and start over again. I also know from my time monitoring my fertility naturally that cycle trackers use markers of these hormonal ebbs and flows to attempt to identify whether a woman is ovulating, and, if so, where she is in the cycle. Body temperatures, for example, rise after ovulation until menstruation, thanks to progesterone. A spike is a good sign that a woman has ovulated. There are a number of apps that try to identify when that happens so she knows when she’s fertile.

But 28 Wellness is different. Until very recently, when it launched a temperature-based fertility tracker behind its paywall, it made no claims about fertility, only management of menstrual symptoms. Whether symptoms can be mitigated without careful attention to all the markers of fertility—the daily thermometer readings, urine tests for hormones, and monitoring of cervical mucus that make fertility awareness as fascinating as it is frustrating—is still an open question. I had my doubts whether the app’s feel-sexy-even-on-your-period pitch could deliver on what I suspected were deceptively simple promises, so I took my questions to the experts in the field of female fertility and femtech.

My first question was whether the app is capable of doing what it says it does: identifying phases of a woman’s menstrual cycle in order to provide tailored eating and workout advice.

For Dr. Naomi Whittaker, an app that bases its advice on the length of a woman’s menstrual cycle has already missed the mark. Whittaker, a board-certified obstetrician and gynecologist, tells me that, contrary to popular belief, the most important part of a woman’s cycle is not menstruation—the bleeding itself—but ovulation, when the egg is released from the ovary. Ovulation can’t be predicted using an algorithm. Even a woman with a generally regular cycle could ovulate earlier or later, or not at all, based on life events or changes in her body.

This app and others like it don’t live up to their promise, she says, because they estimate the date of ovulation instead of trying to pinpoint it as the central factor in interpreting a woman’s cycle, as more robust tracking methods do. The accuracy of estimation for a “regular” period is quite low, and when an app offers the ability to tailor the length of your cycle, estimating ovulation based on period length is a guessing game. This of course will affect the quality of the advice given by the app. Like anything else, the data output is only as good as the data input. While the app has a symptom-logging feature that includes everything from mood to libido to cervical mucus, it doesn’t allow users to connect those signs to what they should signify: healthy ovulation or the lack thereof. And the likelihood that it will accurately identify a woman’s fertile window as a result? Low.

But let’s say you do happen to align with the algorithm. For users who are so fortunate, the second question is whether following 28 Wellness’s regimented approach to nutrition—fermented vegetables in the follicular phase, seafood and broths during the luteal phase, and so on—makes a difference. The answer turns out to be yes and no.

Melissa Buchan, a co-developer of the Chart Neo cycle charting app and a registered nurse and board-certified health and wellness coach, sees benefits in basic nutrition guidance. “There are definitely lifestyle changes that can be applied across the board, whether you have a regular cycle or irregular cycle,” she says. “Any type of advice on a whole foods–type diet is going to be a benefit, right?” Following the advice in 28 Wellness, focusing as it does on nutritionally dense foods, is better than reaching for the Ben & Jerrys during P.M.S.

But the value of such guidelines is limited. “As a woman starts to dial in and really wants to have a more nuanced way to hack her cycle, then that’s where tailored advice really comes into play,” Buchan says. Tailored advice, though, doesn’t necessarily mean eating by menstrual phase. It just means knowing what your body needs at any given time, which is often determined by nutrient deficiencies or underlying health conditions. Women with polycystic ovary syndrome or P.C.O.S. have very different dietary challenges than women with endometriosis. Same for women with irregular periods. The right smoothie recipe is not enough. Not everything is as simple as lifestyle optimization.

Though hormonal imbalances are often the underlying cause of the painful periods, apps like 28 Wellness can’t identify those patterns. This uncertainty is part of what led Dr. Amy Beckley to found the app Proov, which tracks women’s cycles using their hormones.

Beckley has reservations about generalized nutritional advice. Consider, for example, a woman with undiagnosed P.C.O.S., whose ovaries may be insensitive to insulin signals. Because of this, she fails to ovulate but still bleeds. While this looks like and will be identified by a basic cycle tracker as a normal period, it is actually “estrogen dominant” and lacks the normal transition to progesterone after ovulation. If she uses an app that encourages estrogen-promoting foods, she’s at best going to see no change and could even “make it worse” without getting any closer to a diagnosis of her condition. In other words, perfectly timed sweet potato intake is not going to help.

This is why she dismisses 28 Wellness’s tracking methods outright: “You can’t take the menstrual cycle from the textbook and paste it into your app and say, oh, you’re on cycle day thirteen, cool, this is what it looks like: Your estrogen is doing this” and provide eating tips based off of that estimate, she says. Without actually measuring hormone levels, she says, “you have no idea.” You’re just guessing.

What about the exercise stuff? Is it also just guesswork? Is there really any correlation between sculpted “Ballerina Thighs” and painless periods? Dr. Jerilynn Prior, one of the veterans of women’s menstrual cycle research, has studied ovulation and menstruation since the 1970s. She tells me about a study she conducted early in her career at Vancouver University on women who wanted to train for marathons, which they weren’t allowed to compete in at the time. “They said to me, how can we do it safely and in a healthy way?” Prior says. They were familiar with the common claim that exercise, especially too much or of the wrong kind, could negatively affect their cycles. Obviously they wanted to avoid that risk, and Prior wanted to know whether and how exercise affects women’s menstrual periods.

Prior’s study, following data from sixty-six women, nineteen of whom ended up running the marathon, found no ill effects on women’s cycles based solely on the amount or type of exercise. What mattered was very simple: that they ate enough to keep up with the massive caloric expenditures required and trained wisely so as not to injure themselves. As long as they maintained their weight while training, they didn’t lose their period—a common fear among female athletes.

This study and her other research convinced Prior that exactly how a woman exercises is less important than that she does so at all: “There was a decrease in [menstrual] symptoms as women increased their exercise,” she says. She worries that women will take individualized exercise regimens as mandates, not encouragements to exercise in a way that feels good for their body that day. What if the app says you’re in the weight-lifting phase, but it’s a glorious day and you’d rather take a run? “Don’t rely on the app to dictate when you should do what,” she says. The evidence that the precise workout regimen makes a difference (apart from the inherent benefits of exercise) “is simply not there.” And she has an even hotter take: The idea that women have increased libido before ovulation because of high estrogen levels is an old wives’ tale. Women are more interested in sex when they have more energy and when they feel good about themselves. So unless you’re trying to get pregnant or avoid it, forget about timing date night based on the day of your cycle.

As with nutrition, the practice itself seems to matter more than the exact moment it’s performed. So how do you build a better app, one that targets the factors that really affect your cycle? Or how do you know if what you’re looking at on the app store is worth the time or money?

One of the first things you learn when trying to answer these questions is that the app’s aesthetics are not your best guide. The science behind fertility makes sex distinctly unsexy, at least in the glossy-magazine sense of the term. Instead, it’s a crash course in biology: It requires ingesting a lot of scientific information and applying it to your own, often unpredictable body. And it demands the kind of discipline perhaps not best followed in a milkmaid dress, and generally not best achieved by simply trusting an algorithm.

There are varying approaches to tracking your fertility. Beckley’s Proov is one of a number that use regular hormone tests to identify ovulation. Buchan’s Chart Neo, the only H.I.P.A.A.-compliant fertility tracker, allows women to input cervical mucus along with L.H. levels and other markers and then connects users to individual medical professionals for more detailed monitoring and treatment. Prior’s Ce.M.C.O.R tools, available on her website, use her Quantitative Basal Temperature method along with a detailed symptom diary to identify ovulation.

What these methods share is the ability not only to monitor cycle length and log symptoms but to identify a particular woman’s hormonal patterns. Instead of assuming ovulation date and phase length and hoping for the algorithm to work its magic, they attempt to identify variations as meaningful signs of a healthy reproductive system, which is a woman’s “fifth vital sign.” Women don’t always ovulate on day fourteen; just like a heartbeat, fertility levels aren’t constant. That’s the whole point, and a reliable app’s starting point.

Crucially, those who promote these methods also share transparency on their algorithms—what data is going in and how it’s used—and what clinicians and scientists created them. This gives women information to make informed decisions instead of just asking them to trust the app. (28 Wellness’s website does not clearly establish its scientific credentials.)

Depending on who builds them and how, then, app-based trackers can be useful tools. But research suggests that when trusting interpretation of your data to any app, you’re rolling the dice—yes, the data you get out is only as good as the data you put in, but different algorithms will also read that data differently. For example, one study by Dr. Marguerite Duane—a board-certified family physician and cofounder of F.A.C.T.S. About Fertility—in 2016 showed that the estimated date of ovulation varied significantly between two of the more robust fertility trackers. Duane cautions against using algorithm-driven predictive methods alone, comparing them to packing for a trip in January based on last year’s weather instead of this week’s forecast. Even apps that give advice based on past temperatures, hormones, and so on can’t predict whether stress, for example, will delay ovulation.

“I strongly believe women should learn from trained instructors in whatever method they’re interested in,” she says. She compares cycle tracking to working with a nutrition coach: People know eating healthy is important, but sometimes they need guidance to learn what their body needs. Real “hyperpersonalized” cycle tracking, then, is not about a personally tailored app but personal relationships—with your body and with people who have the experience and expertise to help read it. It’s not about following a supermodel’s regimen, but learning how to craft your own. She notes that when women adopt this approach, they’re able to track their fertility more accurately, and they can achieve or avoid pregnancy at least as well as when using common forms of artificial birth control.

This means that the effort to improve femtech is less a question of perfecting the algorithm than it is of enabling women to understand their own menstrual cycles. Women’s bodies are not machines, and treating them that way risks missing the point. Neither the pill’s artificial suppression of a woman’s cycle nor dependence on an algorithm helps her learn to interpret the signals her body sends her about her health.

If an app encourages a woman to start paying attention to her menstrual cycle, then it might be better than nothing. The desire to appear more attractive is what gets many of us interested in these questions about health in the first place. But this pursuit is not synonymous with well-being, whether physical, psychological, or spiritual. Looking hot and being well are not necessarily intertwined experiences. And every doctor I talked to warns me that grounding the pursuit of health in the quest for sex appeal is especially dangerous for young women who already are presented with unrealistic mental and physical ideals everywhere else in their lives.

“I would never recommend that to my twenty-year-old daughter,” Buchan says. “For a young girl who’s trying to navigate the world and to be confident in her own body image, that’s not something that they need to be seeing.”

In fact, comparing yourself with the images in 28 Wellness could make the problem worse. Mental factors like stress wreak havoc on menstrual cycles, as do eating disorders—including orthorexia, an obsession with obtaining perfect health by rigidly following wellness regimens. A healthier approach is not pursuing changing standards of beauty or putting faith in magical health transformations, but working toward an understanding of what fertility is and is for.

As research continues to reveal just how central the menstrual cycle is to a woman’s overall health, apps like 28 Wellness and the experts I talked with are vying over the very definition of a “natural” approach to fertility. This is what’s going on behind all the cute logos on the app store. The territory beyond modern birth control in a high-tech society is uncharted, but for both mind and body, the real health boost happens in the world beyond the app.

After using 28 Wellness for a few days, I participated in a “research study” promoted within the app. Its aim was to demonstrate that women feel better when using its methods. It includes these three questions:

How often in the last three months have you felt that your body was feminine?

How often in the last three months have you felt that your body was sexy?

How often in the last three months have you felt that your body was something to be looked at?

“Women deserve really good science,” Whittaker says. Why do they keep getting love potions instead?