This relies on ovulation timing. Technically, if there is no egg to be released within the next few days, a woman will not be able to conceive. However, using t

This relies on ovulation timing. Technically, if there is no egg to be released within the next few days, a woman will not be able to conceive. However, using temperature as a surrogate end-point for ovulation does not yield a perfect correlation, there will be statistical error and there will be individual differences obscuring the effectiveness of the method. Then there's the issue of people using the app imperfectly and having sex near ovulation anyway.