Objective To examine frequency domain measures of heart rate variability and their circadian rhythms in pregnancy.
Design A longitudinal study.
Setting University hospital in Turku, Finland.
Participants Sixteen healthy women between 11 and 27 weeks of pregnancy; 12 women before pregnancy; and four women postpartum.
Main outcome measures Heart rate variability as measured in frequency domain from 24-hour ambulatory electrocardiography.
Results Pregnancy was associated with a lower standard deviation of R-R intervals (P < 0.01), with reduced very low (P < 0.05), low (P < 0.01), and high frequency (P < 0.05) power spectral components of heart rate variability. The high frequency power was lower at night in pregnancy, but similar in the daytime in pregnant and nonpregnant women.
Conclusions Pregnancy is associated with an overall reduction in heart rate variability, most markedly reflected in the low frequency component. This suggests altered baroreflex or sympathetic modulation of heart rate, and decreased vagal activation at night.