Plasma progesterone concentrations during pregnancy and lactation in Mongolian gerbils (Meriones unguiculatus)

Plasma progesterone (P-4) concentrations during pregnancy and lactation in Mongolian gerbils were measured by enzymeimmunoassay (EIA). The P-4 concentration was found to increase rapidly after mating and reach a peak on day 6 of pregnancy, The maximum level (166.0 to 184.8 ng/ml) was maintained to day 12 and then decreased to a plateau at a moderate level until day 21. The P-4 concentration declined to its lowest level on day 24 of pregnancy, one day before parturition. The P-4 concentration on the first day of lactation was similar to that on the last day of pregnancy. The concentration then increased to a significantly higher level on day 4 of lactation, remained relatively stable until day 19, and then significantly dropped on day 22. The present study in gerbils showed that the plasma P-4 concentration during pregnancy was higher in the first half than in the second half of gestation and that the P-4 level during lactation was fairly constant. The pattern of changes in the P-4 level during pregnancy and lactation is therefore quite different from that of other rodent species, such as rats, mice and hamsters.