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 (P
4) was measured in serial blood samples collected every 3 days throughout gestation period. The P
4 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 P
4 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 P
4 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 P
4 levels among 3 groups according to gestation length, resulting in lower P
4 level from females with longer gestation length. Therefore, difference of plasam P
4 level during the gestation is due to litter size.
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