Objective: We studied the effects of normal pregnancy on heart rate variability as a noninvasive index of maternal cardiovascular autonomic modulation. Study Design: Twenty-four–hour Holter recordings were obtained for 8 healthy pregnant volunteers during early pregnancy (≤6 weeks after conception); at 10, 18, and 34 weeks after conception; and while the subjects were not pregnant. Another 12 subjects underwent 2 to 4 recordings in the sequence. Heart rate variability and nighttime respiratory rates were determined. Results: Maternal heart rate variability declined during pregnancy. Virtually all indexes were significantly decreased with respect to the nonpregnant state in early pregnancy, and respiratory rate was increased. Changes in heart rate, heart rate variability, and respiratory rates between subsequent stages were modest. Individual changes in heart rate, heart rate variability, and respiration during pregnancy varied in both magnitude and direction. Conclusion: Most of the change in cardiac autonomic modulation and respiration during pregnancy occurs within the first 6 weeks after conception. There is considerable variability in individual heart rate and heart rate variability changes during a normal pregnancy. (Am J Obstet Gynecol 1999;180:978-85.)