The relationships between plasma progesterone (P) and the numbers of viable embryos and resorptions were examined in aging C57BL/6J mice to elucidate possible mechanisms contributing to the loss of fertility during aging. Mice of previously proven fertility were killed on each day of pregnancy. In mice aged 3–7 months and 11–12 months, >90% mated successfully. The older mice had 12% fewer implantation sites on Days 6–10: 8.5 ± 0.06 (young); 7.5 ± 0.26 (older); (mean ± SEM), P<0.001. Major losses of viable embryos occurred after Day 13 in older mice, chiefly because the average resorption frequency doubled (P<0.001). In 6% of the older mice, all conceptuses underwent resorption, a phenomenon which was rare in younger mice. Total uterine weight was less throughout pregnancy in the older mice. However, when uterine weight was normalized/viable fetus, there were no differences between the age groups from Day 10 to term. Estimates of the weight of the decidual swellings on Days 6–10 indicated an age related deficit on Day 7 only. Additionally, less uterine intraluminal fluid was found in the older mice on Day 1. Plasma P increased after Day 1 on a similar time course in both age groups. However, the increase in mean P values tended to be less in older mice on Days 1–6; the deficit of 40% on Day 4 was significant (P<0.05). No correlation was found between plasma P and the number of viable fetuses or resorption sites during Days 11–15. The possible relationship of P deficits in older mice during early pregnancy to the subsequent reduction in fetal survival is discussed.
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Holinka et al. (1979) studied this question.