Introduction: The number of older pregnant women and obese people is now increasing.

Objectives: To clarify relations between age and autonomic nervous system activity (ANSA), body weight (BW), and percentage fat mass (%FM).

Methodology: Subjects were women who underwent measurement twice (1 to 4 months, and 5 to 8 months) during the first 8 months after childbirth. Heart rate variability was measured as ANSA with the Inner Balance Scanner. BW and %FM were measured using a DC‐320 body composition meter (Tanita). Data were analyzed by calculating Spearman rank correlation coefficients. The study protocol was approved by the Ethics Review Board of X University.

Results: Mean subject age was 33.0 ± 4.1 years. Mean body weight before pregnancy was 50.3 ± 6.6 kg. Mean weight gain due to pregnancy was 8.1 ± 4.6 kg. Mean weight loss per day from the first to the second measurement was 8 ± 18 g. Decrease in %FM from the first to the second measurement was 0.016 ± 0.024%. With regard to relations between age and ANSA index [Total Power (TP), Low‐Frequency (LF), High‐Frequency (HF), LF/HF], BW loss, and %FM decrease, weak negative correlations were observed in TP, LF, and HF at the first measurement (rs = ‐0.274, rs = ‐0.253, rs = ‐0.255, respectively; p < 0.05) and in LF/HF and %FM decrease at the second measurement (rs = ‐0.264, rs = ‐0.283, respectively; p < 0.05).

Conclusion: With aging, the decrease in postpartum %FM was reduced, as was ANSA associated with energy metabolism. These findings suggest that postpartum %FM management and ANSA activation are particularly important for the prevention of obesity in elderly women.