Progesterone, cortisol and oestradiol-17 beta in the initiation of human parturition: partitioning between free and bound hormone in plasma.

The concentrations of total and free progesterone, oestradiol-17 beta, and cortisol were measured in the plasma of 24 pregnant women from 10-17 days prepartum, during labour, and for 2-3 days postpartum. All the women were delivered vaginally, and their hormonal profiles were similar, whether they initiated labour spontaneously (n = 11) or were induced by rupture of the fetal membranes (n = 13). Before labour, the plasma concentrations of total radioimmunoassayable progesterone, oestradiol-17 beta and cortisol were 557.3 (SEM 70.3) nmol/l and 590.0 (SEM 76.7) nmol/l, 94.3 (SEM 18.7) nmol/l and 111.6 (SEM 19.8) nmol/l, and 1087.2 (SEM 103.7) nmol/l and 1020.7 (SEM 100.7) nmol/l, for the spontaneous and induced groups, respectively. During labour progesterone and oestradiol-17 beta declined in maternal plasma to around 130 nmol/l and 30 nmol/l in both groups by the end of the third stage. The free fractions of progesterone and oestradiol-17 beta remained constant throughout labour and over the entire study period at 2-3% and 1-2%, respectively. Total plasma cortisol rose to 1243.7 (SEM 83.3) nmol/l (P less than 0.02, spontaneous group) and 1247.5 (SEM 97.4) nmol/l (P less than 0.001, induced group) by the end of the first stage of labour. Concomitantly, free cortisol increased from 4.5-5.5% to 7.2-8.2% over this period so that its concentration increased by approximately 2.5-fold in both groups of women during labour. We conclude that the total and physiologically-active free concentrations of progesterone and oestradiol-17 beta in maternal plasma do not change immediately before the onset of labour.(ABSTRACT TRUNCATED AT 250 WORDS)