Changes in autonomic cardiovascular control in mid-pregnancy.

Spectral analysis of heart rate variability was used to study autonomic nervous control in mid-pregnancy. Fifty women (age 22-36 years) with singleton pregnancies (mean duration of gestation 27.7 weeks) and 39 non-pregnant female controls (age 21-39 years) were studied using controlled breathing and orthostatic tests. During spontaneous breathing the overall heart rate variability was lower in pregnant subjects indicating a decreased parasympathetic tone at rest. The decreased parasympathetic tone probably counts for the increased heart rate in pregnancy. The parasympathetic efferent capacity of autonomic cardiac control was found to be similar in pregnant and non-pregnant subjects, as no difference was seen during controlled breathing in periodic heart rate variability between the groups. Standing up caused a similar change in low frequency and mid-frequency bands in both groups, but high frequency heart rate variability increased in pregnant subjects and decreased in the controls indicating an increased sympathetic tone at rest in mid-pregnancy.