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Perhaps you've used a period-tracking app for years and now you're ready to start trying to conceive. Or maybe you've been trying and you're considering downloading a fertility app. It seems simple and clear, all neatly marked on a calendar grid: fertile, or not.

There are over a thousand of these apps available, from simple period trackers to ones specifically designed to help plan or prevent pregnancy. They fall into two main categories: calendar-based, where users input the start and end of their periods (like Flo, Dot, and Clue, the latter of which can be synced with Apple’s HealthKit) and sympto-thermal, based on physical signs of ovulation such as cervical mucus, levels of luteinizing hormone (measured by home ovulation tests) or, most commonly, basal body temperature (the body’s temperature on waking).

Some, including Kindara and Yono, sync with thermometers, while fertility-tracking bracelet Ava collects three million data points as users sleep, measuring everything from skin temperature to resting pulse to blood flow. Natural Cycles, which is certified as birth control by the FDA, is calculo-thermal, meaning it combines calendar and temperature information.

Whatever the specifics, the aim is the same: to identify a user’s fertility window—the six days in which they can conceive, made up of ovulation and the five days preceding it.

So far, research into these apps’ efficacy has focused on preventing pregnancies, as it’s the higher-stakes scenario. But if someone who is trying to conceive trusts an app that miscalculates when they’re fertile, they could miss their chance, month after month.

A 2015 Georgetown University studyinto the use of these apps to avoid pregnancy suggests that’s probable. Researchers looked at 30 apps that predicted a fertility window, 24 of which misclassified fertile days as infertile, and vice versa. Lead author Marguerite Duane, M.D concluded that tech interventions should be used with caution. “While an app can be a wonderful way for a woman to track her data, she shouldn’t rely on it to tell her when she may be fertile.”

Duane is also a family physician and Executive Director of FACTS (Fertility Appreciation Collaborative to Teach The Science), an organization that advocates for fertility awareness education, and recommends that women learn to observe their cervical mucus instead—clear with an egg-white consistency means ovulation is approaching—but that they consult a qualified instructor to ensure accuracy.

Earlier this year, German researchers studied the plausibility of using fertility apps to plan a pregnancy, looking at 12 popular brands, including Clue, Flo, Period Tracker Deluxe, and Natural Cycles. They didn’t test apps’ efficacy but rather looked at the methodologies that underpin them and assessed how accurate their predictions should be. Natural Cycles fared badly, because calculo-thermal apps adjust their fertility window after recording a temperature rise (meaning ovulation has probably taken place), making them more useful for avoiding pregnancy than planning it.

They dismissed calendar-based apps because they calculate future cycles based on previous ones, rather than new information. Three sympto-thermal apps (Lady Cycle, myNFP and Lily) did impress because they’re based on Sensiplan, a family planning method that involves monitoring temperature, mucus, and changes to cervix shape, and is proven to work. But while scientists might understand the intricacies of the information used to forecast fertility, it’s less likely users will.

Some of the most popular apps, including Flo, Clue, Fertility Friend, and Kindara, didn’t respond to Mashable’s request for information on how their algorithms are calculated, while others (Ovia, Ava) were vague about the specifics.

A Natural Cycles spokesperson listed the factors their app considers, which include “sperm survival, variation in cycle length, ovulation day, temperature fluctuations and the length of the follicular and luteal phases” (the pre- and post-ovulation parts of the menstrual cycle). Leslie Heyer, president of Cycle Technologies, which makes Dot, a calendar-based app that provides a fertile window of 12-17 days (erring on the side of caution to ensure no potentially fertile days are missed) cited similar factors. “From the user's perspective, it's extremely simple. However, in terms of what goes into determining her fertility risk, there is a lot of information going in. Anyone would be hard pressed to do that type of calculation independently.”

But does anyone need to, especially when the result contains such a large margin of error? Eve Feinberg, MD is assistant professor of obstetrics and gynecology at Northwestern University and vice president of the Society for Reproductive Endocrinology and Infertility. Most of her patients use some kind of fertility app. “For many women, it's empowering. It gives them information, they feel like they're in control and it educates them about their bodies.” But she questions whether they have any quantifiable medical benefits. “It's not like visits to fertility clinics have declined since the advent of apps.”

Dr. Feinberg says that most couples will conceive within a year, and insists past periods are a good predictor of future ovulation. The key is for women with regular cycles to have sex on days 10, 12, and 14, while others should try every other day after their period. “It shouldn't be hard to hit the window when the egg is released. It's not rocket science.”

Unlike most fertility apps, though, it is free. While basic versions are available at no cost, users can pay from $1.99 a month for period tracking and more than $200 for wearables. No wonder the market for “femtech” (female technology—often focused on women’s reproductive health) is expected to be worth $50 billion by 2025.

Not that femtech companies are only in it for the money. Many seem driven by a genuine enthusiasm for women's well-being. But it would be disingenuous to suggest that fertility apps do significantly more than re-package long-standing family planning methods, sometimes monitoring additional parameters that aren’t proven to be relevant.

Women who want to know when they’re fertile should use whatever approach works for them, but believing any app over their body’s signs would be a recipe for disappointment. “Women are smart,” says Duane. “They are smarter than their smartphones.”

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• Baby sleep gadgets to try if you're really tired and desperate

• 10 apps to help your kids learn to control their emotions

• Wild parenting advice from the first man to win a paternity leave suit

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