Fertility Apps and Websites: Are They Accurate?

Posted on August 16, 2016 by Inception Fertility

Timing is everything. . . . well, maybe not everything

In the recent past, a wide range of websites and apps have also gotten in on the act, using programs to predict the fertile window—when conception is most likely to occur—by prompting a woman to enter her last menstrual period and the length of a typical cycle. Millions of prospective parents have accessed these apps and sites. Unfortunately, nearly 79 percent of fertility apps and 75 percent of websites inaccurately define the fertile window, according to findings reported by New York researchers from Weill Cornell Medical College in NYC and New York-Presbyterian in Queens.1

Fertile window. More than 20 years ago, Allen Wilcox and colleagues described a woman’s fertile window in the New England Journal of Medicine. Given a 28-day menstrual cycle starting on day 1, they calculated that ovulation should occur on day 15—with a fertile window from day 10 to day 15. Today, this is considered the “gold standard” for calculating a woman’s most fertile period.2

Out of sync with science. In the recent study, the researchers looked at 33 of the most popular free fertility apps on both Android and iOS platforms. They also searched for “ovulation calendar” or “fertility calendar” online, focusing on 20 top websites. How many predicted fertile windows that aligned perfectly with the gold standard? Only 1 website and 3 apps. Given the inherent variability of women’s actual cycles, said the researchers, it is possible that these fertile windows are even more inaccurate than what is reflected in the study. In addition, in any one woman, the day of the cycle that she ovulates may vary from month to month, so unless a woman consistently has a 27-, 28-, 29-, or 30-day cycle every month, the app may not be accurate.

About three-fourths of the fertile days predicted by the apps and websites fell within the fertile window of the gold standard. Unfortunately, in some cases, the calendars weren’t even close. Some started the fertility window as early as day 5, while others extended it as late as day 21. Starting too soon is problematic because couples often abstain from sex for several days prior to the fertile window to increase sperm concentration. They might miss the fertile period altogether.

In addition, not all the calendars provided the gold standard’s well-accepted 6-day fertile window. The shortest window was 4 days, and the longest was 12. The only day that was consistent with the gold standard for all the calendars was day 14.

There was a bit more accuracy in predicting the day of ovulation. Of the calendars that predicted a day of ovulation, 80 percent of websites and nearly 87 percent of apps accurately identified it as day 15.

Clinical impacts. Although these apps may help a woman keep track of her menstrual cycle lengths, apps that attempt to predict an ovulation window tend to be inaccurate, mostly due to biologic variability. Ovulation kits testing a woman’s urine for the LH surge (the signal to ovulate about 36 hours later) are likely to be more accurate in predicting actual ovulation.

Overall, these app- and web-based calendars fall short in providing accurate, reliable information prospective parents so desperately need. How much this misinformation affects their ability to become pregnant is unknown, but timing intercourse clearly remains an important way to increase the odds. Unfortunately, many of these apps and websites counsel patients to attempt pregnancy during days that are not conducive to conception.


  1. Setton, R, Tierney, C, and Tsai T. Obstet Gynecol. 2016;0:1–6.
  2. Skwarecki, B. “Most Fertility Apps Miscalculate the Fertile Window.” Medscape. June 13, 2016.

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