OBJECTIVE The third trimester of pregnancy is characterized by a mildly hyperactive hypothalamic–pituitary–adrenal (HPA) axis, possibly driven by elevated circulating levels of corticotrophin releasing hormone (CRH) of placental origin. In‐vitro studies have demonstrated that glucocorticoids and oestrogen stimulate while progesterone inhibits the expression of CRH mRNA and/or protein, suggesting that several potential interactions between the placenta and the HPA axis may exist.DESIGN AND PATIENTS To investigate the detailed pattern of circulating immunoreactive (ir) CRH, ACTH, cortisol, oestradiol and progesterone during the third trimester of pregnancy, plasma samples were drawn serially every 30 minutes from 22 healthy pregnant women (age 32.0 ± 1.1 years, mean ± SE) between the 34th and 36th week of gestation. Ten women had plasma samples drawn between 0800 h and 2000 h (daytime group), and 12 between 2000 h and 0800 h (night‐time group). The hormone concentrations obtained were analysed for pulsatility by the Detect program, for detection of circadian rhythmicity by comparison between the first and second 6‐hour periods within each group by Student'st‐test, and for time‐dependent correlations by cross‐correlation analysis.RESULTS All five hormones were secreted in a pulsatile fashion. There was no apparent circadian rhythm of CRH or oes tradiol secretion, whereas there was a clear circadian rhythm in plasma ACTH, cortisol and progesterone secretion, with the latter in reverse phase (P<0.05). No significant correlations were observed between CRH and ACTH, whereas, as expected, ACTH and cortisol concentrations were strongly correlated with each other over time (r=0.32 and 0.70 at lag time 30 minutes for the daytime and night‐time groups, respectively), with ACTH leading cortisol. A weak positive correlation was observed between CRH and cortisol concentrations for the night‐time group at lag time 0 minute, suggesting that the latter may have a positive effect on the formerin vivoCONCLUSIONS These data suggest that placental CRH, although pulsatile, drives quantitatively the maternal HPA axis in the third trimester of pregnancy in a non‐circadian, non‐pulsatile fashion. The maternal HPA axis is probably driven in a circadian and pulsatile fashion by another major ACTH secretagogue, most likely