Biphasic changes in autonomic nervous activity during pregnancy

To understand the sequential response of the autonomic nervous system to pr egnancy, we studied heart rate variability in 23 first trimester, 23 second trimester and 21 third trimester pregnant women. Twenty non-pregnant women were recruited as controls. Time and frequency domain measures of heart ra te variability in three recumbent positions were compared. We found that no rmalized high-frequency power in the supine position increased significantl y in the first trimester (42.2 (95% confidence interval (Cl) 5.4) nu (norma lized unit); P<0.05) compared with non-pregnant controls (33.0 (6.0) nu), a nd then decreased progressively in the second (27.3 (6.7) nu) and third (21 .8 (6.0) nu; P<0.05) trimesters. The low-/high-frequency power ratio in the supine position decreased significantly in the first trimester (0.8 (0.3); P<0.05) compared with that of non-pregnant controls (1.1 (0.3)) and increa sed progressively in the second (1.5 (0.4)) and third (2.1 (0.8); P<0.05) t rimesters. When the position was changed from the supine to the right later al decubitus, the percentage change in normalized high-frequency power corr elated significantly and negatively with normalized high-frequency power in the supine position in non-pregnant controls (r=-0.56, P=0.01) and in preg nant women in the first (r=-0.44, P=0.034), second (r=-0.68, P<0.001) and t hird (r=-0.68, P<0.001) trimesters. These results indicate that autonomic n ervous activity shifted towards a lower sympathetic and higher vagal modula tion in the first trimester, and changed towards a higher sympathetic and l ower vagal modulation in the third trimester as gestational age increased. The balance between the haemodynamic changes of pregnancy and aortocaval co mpression caused by the enlarging gravid uterus may be responsible for the biphasic changes in autonomic nervous activity during pregnancy.