The Go Backs don't need recognition. I doubt Kahvi recognized Vaya's dad. But they also don't seem to know exactly when they are pregnant, the way other elves do because recognition is the indicator. (Krim is an example. She had no idea she was pregnant until Mender told her.) If Kahvi is able to guarantee odds at all... it probably just has something to do with knowing when she's most fertile. Even human beings can do that.