When my wife and I decided to have our first child, we did basically the reverse of this method (ie, had sex exactly when this method would tell you not to). An

When my wife and I decided to have our first child, we did basically the reverse of this method (ie, had sex exactly when this method would tell you not to). And we were expecting after the first month. So, the predictability is there; it's just the usability that makes this method not a reliable form of birth control.