Prolonged Gestation, Elevated Preparturitional Plasma Progesterone and Reproductive Aging in C57BL/6J Mice1

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Abstract
Gestation is prolonged during aging in previously parous C57BL/6J mice from 18.6 days at 3-7 months to 20.8 days at 11-12 months. Plasma progesterone, measured by radioimmunoassay, decreased sharply after Day 16 of gestation in 3-7 month-old-mice, whereas the rate of decrease in 11-12-month-old mice was 55% less than in the younger group. In 3-7-month-old mice, preparturitional progesterone levels were inversely correlated (P<0.05) with the number of viable fetuses on Day 18, whereas such a correlation (P<0.05) persisted from Days 19-22 in the older mice. In 11-12-month-old mice, gestation length was inversely correlated with litter size; no significant correlation was found in the younger group at first or second pregnancy. Because the decline of progesterone is generally considered necessary for parturition in rodents, the retarded decrease of progesterone in 11-12-month-old mice is likely to be a major factor in their prolonged gestation. In 3-7-month-old mice at first pregnancy, the gestation length was significantly longer (19.0 days) and the litter size smaller (6.5 pups) than at second pregnancy. Average litter size decreased with age in previously parous mice from 7.8 pups in 3-7-month-old mothers to 4.6 pups in 11-12-month-old mothers. Mortality at birth increased with gestation length in 11-12-month-old mice. The influence of litter size on the luteotropic and luteolytic mechanisms during the period preceeding parturition is discussed.