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

Plasma progesterone (P4) concentrations during pregnancy and lactation in Mongolian gerbils were measured by enzymeimmunoassay (EIA). The P4 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 P4 concentration declined to its lowest level on day 24 of pregnancy, one day before parturition. The P4 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 P4 concentration during pregnancy was higher in the first half than in the second half of gestation and that the P4 level during lactation was fairly constant. The pattern of changes in the P4 level during pregnancy and lactation is therefore quite different from that of other rodent species, such as rats, mice and hamsters.