From Creatures Wikia
The Breeder's Kit monitors the sexual chemicals (oestrogen, progesterone, sex drive, testosterone) in your norn's bloodstream, and also tells you if a norn is pregnant, and how many months pregnant she is. Like the Health Kit, there is a page of medicinal herbs and weeds to hand (the Tomato and Ugly Tomato) that have effects on norn fertility.
The big red "X" on the picture of a male norn in the fertility kit simply means that he cannot carry an egg. Don't worry, he can still father a child!