Plasma progesterone, serum estriol and plasma HPL determinations during the last trimester to detect changes before spontaneous labor. Comparison of progesterone assay using RIA and CPB

Conflicting results have been published regarding changes in plasma progesterone during the last trimester of pregnancy. Some have demonstrated a fall in plasma progesterone before labor, and this has been taken as a possible explanation of the onset of labor. It has been suggested that the various results could be due to differences in methods for progesterone determination. In this study the progesterone levels were determined by both RIA and CPB. In 11 women the plasma progesterone, human placenta lactogen, and serum estriol were measured weekly during the last trimester of normal pregnancies and immediately after delivery. All samples were analysed radioimmunologically. In order to compare the radioimmunoassay and competitive protein binding techniques (RIA and CPB), the progesterone levels were determined by both methods. This was also done for 80 successive plasma progesterone routine samples drawn from women who were not pregnant or who were in the early stages of pregnancy. Both methods showed a significant rise in the plasma progesterone level during the last 6 weeks before spontaneous labor. However, the values obtained were lower when assayed by CPB than by RIA, presumably because of a higher specificity and a cross reaction in RIA. Serum estriol exhibited increasing values throughout pregnancy, but without a significant rise during the last few weeks. Plasma HPL settled at a constant level during the last few weeks before labor.