Effects of litter size on gestation length and plasma progesterone concentration in Mongolian gerbils (Meriones unguiculatus)

A total of 31 female Mongolian gerbils (19-54 weeks of age at mating) were used for this study. The mean litter size at birth in all females was 4.4+-0.3, ranging 1 to 8 pups. Although maternal age did not affect to litter size and gestation length, litter size did significantly to gestation length. Plasma progesterone (P4) was measured in serial blood samples collected every 3 days throughout gestation period. The P4 concentration increased rapidly after mating, reaching a peak level on day 6 of gestation, maintained the high level to day 12 of gestation, then decreased to a moderate level and sustained at the level until day 21. The P4 concentration decreased to a lowest level on day 24 of gestation (mean gestation length: 25.2 days). Females having a large number of litter size tended to reveal higher level of P4 during gestation, comparing with that females having a fewer litter size, and significant differences between them were found on days 6 and 9 of gestation. There were significant differences in P4 levels among 3 groups according to gestation length, resulting in lower P4 level from females with longer gestation length. Therefore, difference of plasam P4 level during the gestation is due to litter size