“At first [my husband and I] were worried,” she said, “but then we got used to it and have grown to trust it. I honestly can't imagine ever going back on the Pill.”

Other women told me the apps simplify what they would have been doing anyway. Rachel, who lives in Scottsdale, Arizona, said she’s been using fertility-awareness methods almost exclusively for 10 years. Before apps like the two she relies upon, Clue and OvuView, came along, she kept a three-ring binder full of temperature readings by her bed.

“My husband has been known to pop the thermometer in my mouth when I wake up and remind me to do cervical checks in the evening,” she said. “It's brought us closer than ever because he's so in tune with my body and my natural cycles.”

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Leslie Heyer, president of Cycle Technologies, which makes CycleBeads, told me that its success rate is about 95 percent for “perfect use” and 88 percent for “typical use,” which would mean it beats condoms and falls just short of the Pill. And though women using CycleBeads have only 18 infertile days each month (14 if you deduct menstruation days), Heyer says women using the method have intercourse just as often as those on the Pill.

None of the women I contacted said they had any slip-ups or unintended pregnancies, and some said fertility awareness was just as easy to handle as the Pill or condoms. Rachel even described it as “like breathing.”

However, several of them emphasized that there’s a difference between what they do and the “rhythm method,” a different kind of days-counting technique developed in the 1930s, which doesn't use temperature readings and which has a historical association with the Roman Catholic church.

Also important is the distinction between apps like Daysy’s and CycleBeads, which are specifically designed for contraception, and those that only track ovulation for couples who are trying to get pregnant. (Clue doesn't officially bill itself as birth control, for example.) Experts don’t recommend simply repurposing a basic fertility app for contraceptive purposes.

The American College of Obstetricians and Gynecologists said in a statement that, "Natural family planning is not as effective as most other methods of birth control. One in four women who use this method become pregnant." The organization also said that women who have abnormal bleeding, vaginitis, cervicitis, frequent fevers, or who are on certain medications should not use these methods.

Nathaniel DeNicola, an OB-GYN with the University of Pennsylvania, said natural methods are “definitely not the most effective,” but can nevertheless be a good option for women who are fed up with hormonal methods, leery of copper IUDs, or have health risks that preclude taking the Pill. Because being off by even a few days can result in pregnancy, he says it’s essential women track their days and symptoms vigilantly—and if apps can help with that, so be it.