This study evaluated PA comprising housework and child-rearing in women at 2 months postpartum to examine the relationship between PA and mental health. A total of 30% of the participants had mental health problems, and multiparous women had poor health. Regarding PA, many women were engaged in low-intensity activities (≤ 3 METs), but they spent substantial time on housework and child-rearing. As a result, the amount of their daily activities was at least 3 METs-h/day.
The duration of moderate or vigorous PA recommended for maintaining health is 73 min/day. In both cases, both the WHO and the Japanese standards were met. Sedentary behavior accounted for at least 7.5 h/12.5 h. The relationship between mental health and PA indicated that the amount of daily activities tended to increase when the health status was good, albeit not significantly. In addition, the amount of daily activities exceeded the recommended WHO and Japanese standards regardless of the mental health status.
Participant characteristics
The daily mean number of steps walked by adults worldwide has been reported to be 5,000–8,000/day [
41‐
44]. Among Japanese women (age range, 20–40 years), who walk a relatively greater number of steps than women in other countries, this value has been reported to be 6,755 steps, indicating that the number of steps walked by the participants in this study, who required time to nurse and rest, was slightly lower than the mean [
44,
45].
Walking activity was calculated based on the number of steps walked by the participants using a conversion table [
46]. Accordingly, the walking activity was estimated to be approximately 3 METs-h/day. However, the walking activity estimated using the accelerometer was lower (1.35 METs-h/day). Such differences may be explained by the fact that the accelerometer used in this study recorded PA with an intensity of ≥ 3 METs when a person walked at a constant speed.
These results suggested that women engage in irregular and light walking associated with child-rearing and housework during the postpartum period.
The GHQ total score was similar to that of women at 4 months postpartum [
47]. The cases of health problems accounted for 30% of the total, corroborating the results reported by previous studies focusing on women at 2 months postpartum [
48‐
51]. The mean body mass index of Japanese women at 2–4 months postpartum is 20.1 kg/m
2 [
52], which indicates that most Japanese women do not require a particular form of PA to lose weight after childbirth.
PA
The mean sedentary time (lying down or sitting) during which the device was worn was at least 7.5 h/12.5 h. Previous studies using accelerometers reported that women at 3 months postpartum spent 8.6 h/13 h [
53] and 9.3 h/19 h sitting [
14]; these results were consistent with those of the present study. The time these women spent doing sedentary activities included the time spent nursing. In their study on nursing women at 2 months postpartum, Maehara et al. reported that the mean nursing frequency was 8.9 times/day and the mean nursing time was 36 min/time, indicating that at least 5 h/day was spent (sedentary) nursing, including night-time nursing [
54]. The risk of venous thromboembolism is high due to increased coagulation capacity and changes in the vascular endothelium at 12 weeks after childbirth [
55‐
57]. Hence, women in their postpartum period, particularly those who underwent cesarean section and those with obesity, should be wary of venous thromboembolism. The WHO recommends that women in their postpartum period increase the amount of PA that they engage in to any level of intensity as
doing some PA is better than doing none [
1].
In addition to hydration, it is important for women who are at risk of developing venous thromboembolism to avoid holding the same posture for a long duration during lactating.
The WHO recommends that women engage in at least 150 min/week of moderate (3–6 METs) aerobic exercise and perform activities that strengthen muscles while simultaneously engaging in low-intensity activities of daily living. Housework is also recommended as an important form of PA [
1]. The results of the present study showed that the amount of time corresponding to at least 3 METs of PA exceeded 70 min/day, which can be expected to exceed 490 min/week; therefore, it exceeded the standard recommended by the WHO.
In addition, the Japanese standard for PA is stricter than the WHO standard for PA and the Japanese guideline states that at least 3 METs of PA is equivalent to 23 METs-h/week or 60 min of brisk walking every day [
47].
In the present study, the amount of daily activities (housework and child-rearing) was equal to approximately 3 METs-h/day, which nearly met the Japanese standard. Although the participants spent a significant amount of time remaining sedentary, the results showed that the amount of their PA was adequate.
A study using accelerometers reported that women at 6 months postpartum spent > 80 min/day doing moderate to vigorous PA [
7], which is in line with the results of the present study, although the study periods in the two studies differed. Evenson et al. studied women at 3 months postpartum and reported different results as they used two cutoff points. The cutoff points for PA established by Troiano et al. were 17 and 1 min/day for
moderate and
vigorous PA, respectively, whereas those established by Swartz et al. were 276 and 2 min/day for
moderate and
vigorous PA, respectively [
14]. The outcomes obtained using accelerometers are difficult to compare as the cutoff points, measurement scales, and data used may differ.
Ensuring that the reproductive organs, which changed during childbirth, return to their normal functions through PA in the form of housework and child-rearing alone is difficult, although the women in the present study engaged in adequate PA. The behaviors that these women frequently engage in (e.g., motion of leaning forward and picking up an infant by one’s arms) and postures in which they lift and hold heavy objects may increase abdominal pressure [
58,
59] and prevent the recovery of the pelvic floor muscles. The training of the pelvic floor and rectus abdominus muscles is important and effective in preventing stress urinary incontinence and recovering sexual function [
60‐
62]. Thus, advising women in their postpartum period to include muscle-strengthening activities in their daily activities is necessary [
1].
Relationship between mental health and the amount of PA
The relationship between mental health and the amount of PA was not significant in this study; however, the amount of PA tended to increase when the mental health was good. Similar results were reported by McLearn et al., who showed that mothers with depression engaged in fewer activities of daily living (e.g., showing their children picture books and playing with their children) [
63]. Mothers with favorable mental health tended to actively participate in child-rearing and housework. Particular attention should be paid to the amount of daily activities exceeding the levels recommended by the Japanese guidelines and WHO even in mothers who had mental health problems. Moreover, 70% of the participants in the group with health problems were multiparous women and can be assumed to engage in housework and child-rearing despite being busy and in poor physical condition. Furthermore, at least 85% of the PA performed per week by women at 3 months postpartum comprised housework and caregiving [
64]; thus, women in their postpartum period perform a substantial amount of PA even when sedentary time is included.
Study strengths, limitations, and future prospects
The results of this study are particularly important as they show that PA performed by women in their postpartum period, as measured using accelerometers, satisfies the standards set by the WHO for the amount of daily activities comprising housework and child-rearing. Therefore, these results may be highly useful in postpartum health education in the future. Specifically, the author believes that postpartum women can meet their required PA by incorporating pelvic floor and rectus abdominus muscle training to their normal daily activities. However, this is only limited to women who are not in need of postpartum weight reduction. Further, the amount of daily activities tended to increase in women with favorable mental health after child birth. Such findings will assist in conducting future confirmatory studies.
However, this study has several limitations due to the small sample size. First, the results of this study were not representative of the population. Second, a statistically significant association between the amount of daily activities and mental health could not be demonstrated because it was difficult to find information on sample size determination from previous studies.
To determine an appropriate sample size, power analysis should be performed in future confirmatory studies based on the results of this study as scientific evidence. This exploratory study revealed that many postpartum PAs were low-intensity. Accumulating evidence has shown that low-intensity PA contributes to human health. A confirmatory study that uses postpartum daily activities as the primary endpoint is warranted.