Change of autonomic nervous activity during pregnancy and its modulation of labor assessed by spectral heart rate variability analysis.

PURPOSE OF INVESTIGATION To elucidate the sequential changes of autonomic nervous activity during pregnancy, we examined heart rate variability on two positions and whether autonomic nervous activity affected duration of labor. METHODS Thirty-eight normal pregnant women were studied. Frequency domain parameters (HF, LF, LF/HF ratio) and heart rate were obtained by spectral HRV analysis in the supine and left recumbent position in three trimesters. RESULTS We found HF was significantly higher in early pregnancy, while the LF/HF ratio was significantly higher in late pregnancy. The LF/HF ratio was significantly lower when the left recumbent position was assumed. The LF/HF ratio was significantly higher in the longer labor group of primiparous women. CONCLUSION Our findings demonstrate that sympathovagal balance shifted progressively from a higher vagal modulation towards a higher sympathetic modulation, and the recumbent position activated vagal activity. It is suggested that increased sympathetic activity in late pregnancy could affect the duration of labor.