Diurnal variations in unconjugated oestriol levels during early pregnancy and their relation to maternal cortisol levels.

We studied maternal plasma levels of oestriol (measured by radioimmunoassay) and total cortisol (measured by a protein binding method) in 11 women from 8 to 22 weeks gestation. Blood specimens (5 ml) were drawn over a 24-h period at 60 and 30 min intervals. The diurnal rhythm of cortisol with higher values in the morning than in the afternoon and evening was present at all gestational ages studied. Unconjugated plasma oestriol showed a similar circadian rhythm to maternal cortisol (r = 0.576, P less than 0.01) in the 8th week. From the 9th to the 11th week oestriol values rose and there was no evidence of a circadian rhythm. In the 12th week plasma oestriol again showed a circadian rhythm, with higher values at night (117.3 +/- 9.5 pg/ml) than during day-time (104 +/- 7.5 pg/ml, P less than 0.001). This pattern remained until the 22nd week, when plasma oestriol levels at night (3.78 +/- 0.49 ng/ml) were markedly higher than in the day-time (3.16 +/- 0.3 ng/ml, P less than 0.001). An inverse relation between oestriol and cortisol levels as shown in late human pregnancy could not be demonstrated for the early stages we studied. The interval between rising cortisol and falling oestriol levels decreased from 6 h in the 12th week to 3.5 h in the 22nd week of gestation. This suggests an increasing sensitivity of the fetal hypothalamus to the feedback effect of maternal cortisol.