MouseGenetics Forum > Breeding & Colony Management Thread: "Litter size dropping off a cliff after ~8 months?" --- Post #1 by coloonymgr77 --- Hey everyone, long time lurker first time poster. We maintain a mixed background line (mostly C57BL/6 with some 129 contribution) and I've noticed our breeder pairs that have been together since ~6-7 months are now producing tiny litters (1-2 pups) or nothing at all, whereas the same pairs were giving us 6-8 pups a litter when they started.