Prenatal swimming is a physical activity that uses a swimming pool for pregnant women in the stable period of pregnancy. It has been reported that swimming for pregnant women improves malaise. However, the findings are inconsistent concerning what kind of mechanism improves indefinite complaints. Many studies on autonomic nerves during pregnancy focus on short-term changes. Therefore, we hypothesized that cardiac parasympathetic nervous activity increases during the stable of pregnancy. This study aimed to clarify the changes in the cardiac parasympathetic nervous system from pre-pregnancy to postpartum in a single subject. The subject was a 33-year-old woman. Log HF and heart rate were measured before pregnancy, at 20, 28, 33, and 37 weeks and after delivery. Log HF was measured using the MemCalc method. The subject was kept in the supine position in the water and on land. The measurement time was 5 minutes. Log HF is an index of cardiac autonomic nerve activity. The Log HF in the water condition was higher than in the land condition before pregnancy, showed a lower value at the 20th week of gestation, and changed to a higher value at the 28th, 33rd, and 37th weeks of gestation. HR tends to be lower in the water condition than on the land condition. After 28 weeks, the cardiac parasympathetic nervous system was found to be effective in alleviating malaise in pregnant women. We obtained the following three findings. First, in water floating, there is a period of reversal of cardiac autonomic nerve activity during pregnancy. Second, in water immersion, cardiac autonomic activity predominates after 28 weeks of gestation. Third, cardiac autonomic nervous activity in water is high after 28 weeks of gestation. Participation in the experiment was obtained with the permission of the attending physician.
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