Low fertility in vivo resulting from female factors causes small litter size in 129 inbred mice.

Purpose: 129 inbred mice show poor reproductive ability, as evidenced by small litters; however, the exact cause of this is unknown. In the present in vivo study we examined fertility and subsequent post-implantation development in an attempt to clarify the cause of small litter size in 129 mice.

Methods: 129 or C57BL/6J females that displayed vaginal plugs 1 day after mating with males of the same strain were examined for the presence of fertilized eggs.