My mouse had given birth. It was a 'surprise' thing meaning i got her back ( a single mouse) from the pet store and 2 weeks later out popped 13 healthy babies. The babies have just reached their thirteen week. When should i seperate the boys? AND also, i noticed that she is getting fatter...not necessarily bigger, but rounder all round. Is it possible for a mouse to get pregnant without the presence of a male, simply by 'storing' sperm cells from the first mate? Or perhaps it is just a 'post-birth' thing. Also, might there be any possibility that her young had impregnated her? I was under the impression mice only reach sexual maturity at age 4 weeks.