SERIAL MEASUREMENT OF ARTERIAL PLASMA PROGESTERONE LEVELS THROUGHOUT GESTATION AND PARTURITION IN INDIVIDUAL RATS

Abstract

Daily blood samples were taken from 6 chronically cannulated, fully conscious rats to measure plasma progesterone [P4] levels throughout gestation. P4 levels in individual rats fluctuated by up to 28 ng/ml per day, but tended to be consistently higher or lower than the group mean. The accuracy of predicting P4 levels in individual rats from previous values was examined. P4 levels on day 7 of gestation were negatively correlated with fetal weights near term. There was little indication that high P4 levels at any stage of gestation lead to increased fetal or placental weights. P4 levels on day 17 were positively correlated with the number of corpora lutea, but there was little relationship between P4 and the number or total mass of the placentas. Serial blood samples taken from a 2nd group of 6 rats at 2 hourly intervals showed that the time between the major fall in P4 levels to below 12 ng/ml and the onset of parturition was relatively constant (varying by only 8 h) despite a 29 h range in the total length of gestation.