Genetic and litter size effects on serum placental lactogen in the mouse.

Placental lactogen (PL) and progesterone are important hormones of pregnancy in the mouse. The purpose of this study was to investigate the influence of genetic differences and litter size on serum PL levels in the mouse. Serum progesterone levels were also measured in some experiments. Two features of serum PL levels under genetic control were identified: 1) the gestational profile of serum PL and 2) absolute serum PL levels. Breeding combinations that resulted in profound effects on serum PL levels were without significant affect on serum progesterone levels. Serum concentrations of PL and progesterone were directly proportional to litter size. Conceptus number was found to significantly affect ovarian progesterone release. The presence of uterine tissue antagonized the luteotropic actions of the conceptus but did not affect serum PL levels.