None of my females are huge but one feels pregnant. Her belly is firm versus the other female who's belly is very soft. This female with the firm belly hadn't been breeding for 2 weeks so I was so sure she was ready to pop. However, the other day, she was seen several times being mounted by the male. The other female has pretty predictable heat cycles that I can see. She usually breeds the male every week, week and a half. I'm just at a loss to how to get these rats to breed.