Circadian rhythms in maternal plasma cortisol and estriol concentrations at 30 to 31, 34 to 35, and 38 to 39 weeks' gestational age.

In order to examine the relationship between circadian rhythms in different steroids, cortisol and estriol were measured by radioimmunoassay in maternal venous plasma taken at 30- to 60-minute intervals over 24 hours from nine healthy women at 30 to 31, 11 at 34 to 35, and 10 at 38 to 39 weeks' gestation. Maternal cortisol was maximal at 0730 to 0830 hours and fell significantly to its lowest concentration between 1830 and 0130 hours. At 30 to 31 and 34 to 35 weeks there was a significant circadian rhythm in estriol concentrations which was inversely related to the circadian rhythm in cortisol. At 38 to 39 weeks mean estriol concentrations were significantly higher than those at 30 to 31 and 34 to 35 weeks but there was no significant circadian pattern. We concluded that there is an inverse relationship between maternal cortisol and estriol at 30 to 31 and 34 to 35 weeks, which would be consistent with a maternal effect on fetal adrenal function. The increase in mean concentration and absence of circadian rhythm in maternal plasma estriol at term might suggest an additional positive stimulus to fetal adrenal function. The existence of patterns in maternal estriol concentrations must be taken into account in the u;se of sequential measurements of this hormone during the last 10 weeks of pregnancy.