Fluctuations in water vole population size depend on abiotic and intra-population factors affecting the physiological condition of females. The relationship between variability in reproductive success and morpho-physiological characteristics of female during pregnancy is studied quite poorly. In standard vivarium conditions, the morphometric and hormonal characteristics of female were assessed at different stages of pregnancy (first trimester--days 4-7, second trimester--days 8-14, third semester--days 15-20), and their relationship with potential and actual fecundity and the level of embryonic lethality was elucidated. The general regression model was used in the data analysis. Positive correlations were found between potential fecundity and the female body mass at mating, body mass index and blood testosterone level. The reproductive parameters under study were independent of the blood thyroxin level. A positive correlation was established between the level of embryonic loss and the indices of liver and lung functions. Liver and spleen are essential for the maintenance of the female body mass homeostasis during the reproductive period.