BIOLOGY OF REPRODUCTION 12,400-407 (1975) Copyright #{174}1975 by The Society for the Study of Reproduction. All right of reproduction in any form reserved. Pregnancy in Mice Selected For Small Litter Size: Reproductive Hormone Levels and Effect of Exogen

Concentrations of mouse plasma LH, FSH. prolactin (PRL) and progesterone were measured throughout pregnancy in a small litter size line (CN-) in which prenatal mortality is markedly increased especially at the postimplantation stage. The pattern and magnitude of LH and PRL concentrations in line CN- were not significantly different from those previously reported in a control line (C). Levels of FSH and progesterone, however, were lower in the CN- than in line C during the second half of pregnancy. Gonadotropin administration from days 6 through 12 was ineffective in reducing prenatal losses in line CN-, whereas progesterone treatment reduced fetal losses to the level found in line C. Litter size in polytocous species is depend-ent upon the number of ova shed and the proportion of these represented by normal fetuses at birth. Variations in litter size have occurred in mouse strains selected for ovula-tion rate (Falconer, 1960), directly selected for litter size (Falconer, 1960: Bateman, 1966), and as a correlated response to selec-tion for growth rate or body size (MacArthur