So earlier I said maybe it's an oestrous cycle. That's different than a menstrual cycle. Primates have menstrual cycles. Most other mammals have oestrous cycles. Those kinds of cycles may or may not involve bleeding. Dogs have them and bleed- but many other animals have them and don't bleed. Instead of sloughing it off (as happens with us each month), I think the body reabsorbs the tissue that preps it for pregnancy. Maybe GoBacks just have infrequent oestrous cycles, and reabsorb their tissue. And if they don't have sex during that time, they don't get pregnant. GoBacks women, I guess, had sex before every battle- and battles all the time- so there were probably more hits than misses. But maybe Krim and Kahvi didn't get preggers because their timing was off with Cutter and Skot and Pike. Who knows? Maybe Krim did have a few babies- more than Cheiper- that didn't make it, and we just didn't see. Maybe Kahvi wasn't constantly having sex with Cutter. He didn't seem that into it anyway.