Effects of concurrent pregnancy and lactation on reproduction in four strains of mice.

The influence of concurrent pregnancy until the third lactation on reproduction was studied in four strains of mice (SHN, SLN, C3H/He and GRS/A) as a possible step to evaluate the efficiency of this system on offspring production. In all strains, little difference was observed between sequential and concurrent pregnancy and lactation groups in any parameter as the index of reproductivity--delivery interval, litter size, average weight and growth rate of pups, mother weight, rearing rate, still-birth rate and rate of still-born pups. Also, no difference was seen between groups of any strain, in the number of females which had three litters. The estimated number of pups weaned per female during 50 days after first being placed with males was higher in the concurrent pregnancy groups than in the sequential pregnancy groups of all strains. Meanwhile, the incidence of spontaneous mammary tumors, the major characteristic of these strains, was consistently lower in the concurrent pregnancy group in all strains. These results have demonstrated that the concurrent pregnancy and lactation system is an efficient means of mouse production.