If you're trying to get pregnant, there are better ways to predict your fertile window.
Woman looking at her smartphone
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When it comes to keeping tabs on your health, it seems there’s a high-tech solution for everything. But according to a new study in this month’s issue of Obstetrics & Gynecology, most sites and apps that promise to help users track their fertility are “generally inaccurate and unreliable”—and could be hurting more than they’re helping.

Researchers used searches in Google and Apple's App Store to identify the most popular fertility-tracking websites and smartphone apps. Then they plugged in stats for an imaginary user with a 28-day cycle, and four days of menstruation, and compared the results from each tracker to the “gold standard” method for determining a woman's fertile window, established by research back in 1995.

The trackers should have identified the most fertile days as cycle days 10 to 15—the five days before ovulation and the day of ovulation itself. But of the 20 sites and 33 apps examined, only one site and three apps got it right.

“We were extremely surprised by what we found,” lead author Robert Setton, MD, an ob-gyn resident at Weill Cornell Medical College, told Health in an email. The correct website was babymed.com, he said, and My Days, iPeriod, and Clue were all accurate apps, “though the predicted ovulation date of [Clue] was off by one day,” Dr. Setton added.

Other sites and tools were off by as much as 60%. So what’s a woman who wants to get pregnant to do?

Valerie Baker, MD, medical director of the Stanford Fertility and Reproductive Medicine Center and chief of the Division of Reproductive Endocrinology and Infertility at Stanford Medical Center, offers this advice: “We suggest that couples begin to have intercourse four to five days before the anticipated day of ovulation, and continue to have intercourse every other day, or daily until one to two days after the expected date of ovulation based on cycle length."

Concerned about predicting the right dates? You might consider using an ovulation predictor kit, Dr. Baker said in an email. These kits detect the changes in saliva or urine that occur in the days before ovulation.

For women who aren't ready to break up with their site or app of choice, Dr. Setton recommends a dose of healthy skepticism: “I think that women may still use these apps as a general guide to map out their cycles, but they should use [them] with caution, and should certainly try to apply the ['gold standard'] formula to their cycle."

If you've been trying to conceive without success for at least six months, you should consult with your ob-gyn, he said.