OBJECTIVES: To test the reactivity of the autonomic nervous system (ANS) of women in different stages of pregnancy to a psychosocial stressor. DESIGN: Psychosocial stress was induced by a standardized test, the Trier Social Stress Test. SETTING: University of Zurich, Switzerland. POPULATION: Fifty-five healthy pregnant women (28 in the second and 27 in or close to the third trimester) and 24 non-pregnant women in the follicular phase of the menstrual cycle. METHODS: Heart rate variability (HRV) was recorded before, during and after the test. Subjective stress perception was measured by a visual analog scale. MAIN OUTCOME MEASURES: Heart rate, HRV components (high frequency, HF; low frequency, LF, and LF/HF ratio), subjective stress ratings. RESULTS: HF significantly decreased and LF/HF ratio increased during psychosocial stress in all three groups. LF values indicated no significant reaction to psychosocial stress, but a trend towards significance with group interaction was seen. There were no significant differences in subjective stress ratings between the groups. CONCLUSIONS: ANS activity during psychosocial stress seems to be decreased in proceeding pregnancy, whereas subjective stress perception does not show significant changes. The decline of HF due to stress compared to non-pregnant women is not attenuated, just on a lower level.