OBJECTIVE To determine the effect of maternally administered thyrotropin-releasing hormone (TRH) on fetal heart rate (FHR) pattern and fetal breathing movements (FBM). METHODS Prospective observational study of 75 pregnant women between 26 and 34 weeks' gestation in whom pharmacological fetal lung maturation was clinically indicated. Forty-minute recordings were made of FBM or FHR patterns before and after drug administration. Twenty-five received TRH 400 microg as an intravenous bolus, 25 TRH 400 microg in 50 ml 0.9% saline as an intravenous infusion, and 25 acted as controls. Recordings were processed digitally to calculate the change in FHR (n = 45) and FBM parameters (n = 30). The main outcome measures for FHR were number of accelerations and decelerations, baseline rate, overall and short-term variation and duration of high and low variability, while for FBM they were rate, breath-to-breath interval and incidence. Results between groups were compared by analysis of variance. RESULTS There was no significant change in FHR, accelerations or variation in any of the groups. Similarly, there was no change in the incidence of FBM. TRH administered as a bolus produced a small statistically but not clinically significant increase in breathing rate (mean delta = 35 breaths/h, p = 0.004), which was not seen in the TRH infusion and control groups. CONCLUSIONS Maternally administered TRH as used to enhance fetal lung maturation has no clinically significant direct effect on FHR or FBM patterns.