If you rely on a fertility app to track your cycle bad news: Most of them miscalculate the fertile window according to a recent study. The study included the top 20 fertility websites found using a Google search and the 33 most popular fertility apps in the Google Play store and the Apple App Store. Usually a woman with a standard 28-day menstrual cycle will ovulate around day 15 which would also be the last day of a six-day fertile window. While most reliably predicted the day of ovulation the research found that only four provided the correct fertile window.
