Describing itself as ‘confident, scientific and not pink’, Clue uses a special algorithm that takes in to account both global statistics and the information you entered to determine your fertility window and the best days to conceive. The longer you use the app, and the more information you enter, the better the recommendations get. Essentially, the app gets to know you, and that allows it to better “forecast” your fertility. Best of all, you can ask Clue questions.