Changes in the diurnal rhythms of plasma cortisol in women during the third trimester of pregnancy.

We have examined whether there are changes in the day-night variation in plasma cortisol in normal women during the third trimester of pregnancy. Maximum cortisol values at 07.30-10.30 h were similar at 30-31, 34-35 and 38-39 weeks of pregnancy. The daily increment in plasma cortisol (fold change) was greater, and the trough values lower at 30-31 than at 38-39 weeks. The time taken for plasma cortisol to fall to 50% of the morning maximum values was longer at 38-39 weeks than at 30-31 weeks of pregnancy. These changes in cortisol rhythms may have implications with respect to alterations in circadian patterns of maternal steroid concentrations as well as fetal biophysical functions.