To evaluate the validity of fertility web sites and applications (apps) by comparing the predicted fertile window of these modalities to the actual fertile window of a standard 28-day cycle.
This was a descriptive study. The top resulting free web sites and electronic apps downloadable to a cellular phone that provide calendars for fertility and ovulation prediction were assessed. Cycles were standardized to 28 days in length, 4 days of menses, and the last menstrual period was set to January 1, 2015. The predicted date of ovulation and fertility window generated were compared with an actual estimated date of ovulation on cycle day 15, January 15, and a fertile window consisting of cycle day 10 to cycle day 15, the day of ovulation plus the preceding 5 cycle days, January 10–15.
Data from 20 web sites and 33 apps were collected. Of all the web sites and apps used, one web site and three apps predicted the precise fertile window.
Web sites and electronic apps used by the general public to predict fertile windows are generally inaccurate, although the clinical effect of this inaccuracy is unknown. Although they all include the most fertile cycle day, the range of the fertility window varies widely. Patients who are trying to conceive with the assistance of calendars generated from web sites and electronic apps should be counseled on the inaccuracy of these modalities.