Regular physical activity is recommended for pregnant and postpartum women for maternal, fetal, and neonatal well-being (1,4,10,30). Health benefits of physical activity during and immediately after pregnancy include possible prevention of gestational diabetes, preeclampsia, and chronic musculoskeletal conditions; support of healthy weight; and improved mental health (1,4,10,19,30). Moreover, regular exercise helps maintain cardiorespiratory fitness levels throughout pregnancy and can facilitate postpartum recovery (4,10,19). Thus, for the many short- and long-term health benefits of physical activity, it is important to study physical activity behavior among pregnant women. Pregnancy is a life-changing event that can initiate an adverse change in physical activity.
Previous retrospective and prospective studies indicate that physical activity among pregnant women declines for recreational (9,15,20-22,25,34,35), occupational (9,20), and overall (9,29) physical activity. The largest changes occur in the duration and intensity of physical activity in the third trimester as compared with the activity levels at prepregnancy or during the first trimester (9,14,21,22,25,28,33,35). It seems that women replace strenuous activities with lighter-intensity activities as their pregnancy progresses, which leads to increased duration of light activity (9,28,37) or decreased total volume of activity (21,28). The scientific literature has concentrated on reporting exercise or recreational physical activity during pregnancy, whereas occupational, household, or transportation activity is often not included in the same study, with only a few exceptions (7,9,20,33).
The American College of Obstetricians and Gynecologists (ACOG) recommends that pregnant women engage in moderate-intensity exercise for at least 30 min on most, if not all, days of the week (1). We found only three studies (12,25,26) that documented the percentage of pregnant women reaching physical activity recommendations, although none of them used the ACOG definition of activity, and all three studies measured only recreational physical activity, ignoring other types of activity that may contribute to health. Only Pereira et al. (25) provided information on the change in percentage of recreational activity during pregnancy and on which gestational week the activity level was measured.
Most previous studies describing physical activity during pregnancy have various limitations, including retrospective or cross-sectional design, small sample size, or inadequate measurement of activity (6,8,16,28). Measurement of physical activity should assess frequency, duration, and intensity in overall activity and in different modes such as in occupation and transportation (23). To address these limitations, we investigated physical activity among pregnant women, documented whether these women reached the recommended levels of activity, and explored how these patterns changed during pregnancy.
METHODS
This study was part of the third phase of the Pregnancy, Infection, and Nutrition Study (PIN3), a prospective study that examined whether physical activity or stress was associated with preterm birth. The participants were pregnant women seeking services from prenatal clinics at the University of North Carolina Hospitals (Chapel Hill, NC) and were identified by study staff through a review of all medical charts of new prenatal patients. The University of North Carolina Institutional Review Board approved the study protocol, and the participants gave their written informed consent. Study staff recruited the women at their second prenatal visit if they were less than or equal to 20 wk of gestation. Exclusions for the recruitment included women under the age of 16 yr, non-English speakers, those not planning to continue care or deliver at the study site, women carrying multiple gestations, or women who did not have a telephone from which they could complete phone interviews. Recruitment began in January 2001 and ended in December 2005. During this time, 3203 women were eligible for the study and 2006 (63%) were recruited. For analyses in this study, we excluded women if they participated in the PIN3 for a second or third time (n = 131), were under the age of 18 (n = 33), or had missing information on physical activity from the two interviews (n = 360), leaving 1482 (73.9%) women in the analysis sample.
Women participated in two research center visits and telephone interviews, along with filling out several self-administered questionnaires. The telephone interviews were carried out at 17-22 and 27-30 wk of gestation, falling into the second and third trimesters of their pregnancy.
Physical activity measurement.
Interviewers administered a past week recall questionnaire on physical activity during the two telephone interviews. The questionnaire assessed frequency and duration of all physical activities separately for recreational, occupational, transportation, child and adult care, and indoor and outdoor household activity. Using recreational activity as an example, the question asking about participation in particular modes of physical activity was "In the past week, did you participate in any nonwork, recreational activity or exercise, such as walking for exercise, swimming, or dancing, that caused at least some increase in breathing and heart rate?" If the participant responded "Yes," then the interviewer asked her to list all types of activities, one by one, with the following question: "What type of recreational activities did you do during the past week?" For each activity, the participant reported the number of sessions per week, the duration of each session, and the perceived intensity level using the following options: "fairly light," "somewhat hard," and "hard or very hard." The perceived intensity categories were developed from the Borg and Linderholm (5) scale. In addition, the activities were later assigned an absolute intensity level using published MET tables (2,3). These questions were repeated for occupational, transportation, child and adult care giving, indoor household, and outdoor household activity. The outcome variables were estimates of the total number of hours in the past week (h·wk−1) and the total number of MET-hours in the past week (MET·h·wk−1), in which MET-hours per week was based on established MET intensities (2,3).
Women were assigned an activity status based on whether they reported any physical activity at the 17-22 and the 27-30 wk of gestation. Those who were active at both trimesters were defined as "active" and those not active at both trimesters were "inactive." Those women who stopped physical activity between the two time points were categorized as "became inactive." If the participant was inactive in the second trimester and active at the third trimester, she was categorized as "became active."
The women were categorized by whether they reached the recommendations for physical activity based on the guidelines by the ACOG (1), the Centers for Disease Control and Prevention (CDC) (24,36), and the American College of Sports Medicine (ACSM) (24,27). The ACOG and the CDC/ACSM guidelines suggest 30 min or more of moderate-intensity activity on most days of the week, but they differ on the type of activity, as ACOG recommends only exercise and CDC/ACSM recommends any type of physical activity. The ACSM's vigorous recommendation includes any type of activity that is vigorous and is carried out at least 20 min, three times per week. We created four recommended activity levels: 1) ACOG's recommendation using an absolute intensity of 4.8-7.1 METs (moderate intensity based on age 20-39 yr) (27); 2) ACOG's recommendation using "somewhat hard" perceived intensity to match with a moderate intensity; 3) combined CDC/ACSM's moderate and ACSM's vigorous recommendation using absolute intensity of ≥4.8 METs; and 4) combined CDC/ACSM's moderate and ACSM's vigorous recommendation using "somewhat hard" and "hard or very hard" perceived intensity.
The physical activity questionnaire took approximately 10 to 20 min to complete. Intra- and interinterviewer quality control measures, such as expert review of taped interviews, were established to ensure that interviewers were asking questions reliably and systematically. The test-retest reliability of this questionnaire was measured among 109 women within 48 h of interview completion at 17-22 or 27-30 wk of gestation. The measures used for this study generally displayed substantial agreement using Landis and Koch's (18) classification. For example, the intraclass correlation coefficient was 0.83 (95% confidence intervals [CI] = 0.76-0.88) for total activity in MET-hours per week. The criterion validity of this questionnaire was examined in 177 pregnant women who wore an accelerometer for 1 wk, kept a daily structured diary, and following these two measures completed a 1-wk recall of the PIN3 physical activity questionnaire. The diary generally displayed moderate to substantial agreement with the questionnaire; the Spearman correlation coefficient was 0.67 (95% CI = 0.55-0.78) for total activity in MET-hours per week. The agreement between the questionnaire and the observed accelerometer data was lower, with the Spearman correlation coefficient of 0.29 (95% CI = 0.10-0.47) for total activity in MET-hours per week comparing total counts. A detailed description of how the physical activity questionnaire was coded and how the recommended levels of activity were derived is available elsewhere (11).
Other measurements.
Women were asked during the telephone interview about their race and ethnicity, marital status, working status, education, parity (live plus still births), and general health. Prepregnancy self-reported weight and height were collected at the recruitment interview at 15-20 wk of gestation for the determination of body mass index as weight in kilograms divided by height in square meters.
Statistical methods.
The percentages and medians with interquartile ranges (IQR) of physical activity at the two interviews were reported separately for fairly light, somewhat hard, and hard or very hard perceived intensity levels in hours per week and for absolute intensity levels in MET-hours per week in each mode of activity. Poisson regression models with generalized estimating equations (GEE) for repeated-count measures, using an exchangeable working correlation, were applied to test whether the change in physical activity (total or by perceived and absolute intensity levels) across the two time points was different (38). The goodness-of-fit statistics of all models indicated overdispersion; therefore, the Pearson scaling adjustment was applied.
The number of reported activities, the MET values of activities, and the duration of activities at 17-22 and 27-30 wk of gestation were reported separately for modes and perceived intensities of physical activity using medians with IQR and were tested for differences in time with the nonparametric sign test. Participation in the modes of physical activity was reported across the trimesters using medians with IQR separately for "active," "became active," and "became inactive" women. We used the Kolmogorov-Smirnov test to explore the differences in the three groups ("active," "became active," and "became inactive"). The signed rank test was used to explore the difference in activity between the 17-22 and the 27-30 wk of gestation among the "active" women. The percentage of women who reached the ACOG and the CDC/ACSM recommended level of activity was reported across the second and the third trimesters. The difference in the percentage was tested using the GEE model as previously described. The SAS statistical package (version 9.1; SAS Institute Inc., Cary, NC) was used for all of the analyses.
RESULTS
Description of sample.
Among the sample of 1482 pregnant women, the median age at conception was 30 yr and most (60.2%) belonged to the age group of 26-34 yr. The majority, 71.6%, were non-Hispanic white women, whereas 17.3% were non-Hispanic African American and 11.1% of other race/ethnic background. The women reported a median of 16 yr of education, and 63% had at least 16 yr of education. Most women had no children (49.4%) or one child (32.9%) and had an excellent (32.0%) or very good (43.3%) self-perceived health status. The median prepregnancy body mass index was 23.4 kg·m−2.
Modes and components of self-reported physical activity.
The majority of women reported some physical activity during the second (96.5%) and third (93.9%) trimesters (Table 1). When examining all modes together, the level of physical activity decreased between the second and the third trimesters in fairly light (P < 0.001) and hard or very hard (P = 0.02) intensity categories, but also in total activity (P < 0.001 for both h·wk−1 and MET·h·wk−1). In the separate modes, decreasing activity levels were observed for fairly light (P = 0.01) and total (P < 0.001 for both h·wk−1 and MET·h·wk−1) care-related activity; for fairly light (P = 0.003) and total (P = 0.05 for h·wk−1 and P = 0.03 for MET·h·wk−1) household-related outdoor activity; and for somewhat hard (P = 0.03), hard or very hard (P = 0.01), and total (P = 0.01 for h·wk−1 and P < 0.001 for MET·h·wk−1) recreational activity. Among the full sample, the number of women who reported any occupational physical activity at their second trimester (32%) and third trimester (30%) remained stable, but they reported very low levels of occupational physical activity, with medians of 0.0 h·wk−1 and 0.0 MET·h·wk−1 for all intensity levels at the second and the third trimesters.
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TABLE 1: The modes and self-reported intensities of physical activity at 17-22 and 27-30 wk of gestation among all women (n = 1482). The reported values are percentages reporting any activity and medians with interquartile ranges (IQR) in hours per week (h·wk−1) and MET-hours per week (MET·h·wk−1).
Care-related, household-related indoor, and recreational activities represented the largest part of women's activities, as each mode constituted around 25% of total physical activity in hours per week (Fig. 1). When examining the total MET-hours per week, where intensity is accounted for, recreational physical activity was the largest contributor, with around 30% of the total MET hours per week. The distribution of percentages in each mode of activity remained fairly stable between the two interviews (Fig. 1), whereas the total time declined between the two interviews (Table 1).
FIGURE 1: Distribution (%) of modes of physical activity in hours per week (h·wk−1) and in MET-hours per week (MET·h·wk−1) at 17-22 and 27-30 wk of gestation.
The women reported on average five different types of activities in the past week in the second trimester and four activities in the third trimester (Table 2). The median MET values of the reported activities did not meaningfully change from the second trimester (4.5 METs) to the third trimester (4.6 METs). The median duration of reported activities significantly declined from 1.1 h per activity at the second trimester to 1.0 h per activity at the third trimester. Women commonly remained active in occupational activity (80%), household-related indoor activity (50%), and recreational activity (48%) between 17-22 and 27-30 wk of gestation (Table 3). There were only 14 (1%) women who reported being entirely physically inactive at both time points. The consistently active women reported higher levels of physical activity for all modes of activity than those switching their activity status during the time points, except for occupational and household-related outdoor activity.
TABLE 2: Reported number of activities, MET values of reported activities, and duration of activities overall and by mode among pregnant women (n = 1482). The values are medians (with interquartile range, IQR).
TABLE 3: Participation in any physical activity stratified by whether women remained active, became active, or became inactive at 17-22 and 27-30 wk of gestation. The reported values are medians with interquartile ranges (IQR) in hours per week (h·wk−1) and MET-hours per week (MET·h·wk−1).
Recommended physical activity.
The proportion of women reaching the recommended level of physical activity using the ACOG definition at the second trimester was 3% for absolute intensity and 13% for perceived intensity (Table 4). When using the CDC/ACSM moderate/vigorous activity definition, the proportion was 15% for absolute intensity and 38% for perceived intensity. The corresponding proportions mostly decreased at the third trimester to 3% (P = 0.9) and 11% (P = 0.05) for the ACOG and 11% (P < 0.001) and 34% (P = 0.005) for the CDC/ACSM moderate/vigorous definition.
TABLE 4: The definitions and percent of the recommended level of physical activity at 17-22 and 27-30 wk of gestation by the American College of Obstetricians and Gynecologists (ACOG), the Centers for Disease Control and Prevention (CDC), and the American College of Sports Medicine (ACSM).
DISCUSSION
On the basis of our large prospective cohort, with a detailed assessment of physical activity, we found that the overall physical activity level slightly decreased between 17-22 and 27-30 wk of gestation, particularly in duration and volume of care, outdoor household, and recreational activity. The care, indoor household, and recreational activities constituted the largest proportion of the total activity level. The women who were consistently active throughout pregnancy reported higher levels of activity in all activity modes than those who became active or inactive during pregnancy. Only a few women reported being entirely inactive during pregnancy. Most women continued working during pregnancy and had sedentary jobs with no or little activity that was reported as at least fairly light. The prevalence of women who reached the recommended level of physical activity varied largely based on the definition and intensity used, although by all definitions, the majority of women did not reach the recommended level of activity.
Methodological issues.
This study used self-reported levels of physical activity, which are subject to recall bias. To reduce potential recall bias, we included telephone interviews and a recall of the past 7 d, both of which are believed to produce better recall than postal surveys and recalls over longer periods (17). Among a sample of pregnant women, the test-retest reliability of the questionnaire and the comparison of the questionnaire to a 1-wk diary were generally substantial in agreement. However, the comparison of the questionnaire with the accelerometer displayed a lower agreement, a pattern similar to what others have shown (8). The MET values assigned for each activity were not specific to pregnant women, but rather standardized for an average adult, which likely underestimated the activity assessed in MET-hours per week, especially in late pregnancy. We used the moderate (4.8-7.1) and vigorous (>7.1) MET categorization based on 20- to 39-yr-old women to create definitions of moderateand vigorous activity (27) rather than using the categorizations provided by the US Surgeon General's Report (36) for an adult (moderate 3-6, vigorous >6 METs). However, our sample did include 37 (2.5%) women 40 to47 yr and 44 (3.0%) under 20 yr who may have been misclassified.
Another limitation of this study was the generalizability, a common concern among clinic-based studies of pregnant women (31,32). Analyses carried out using the PIN1 and the PIN2 that preceded this PIN3 suggest that underrepresented women were more often less educated, younger, African American, had higher parity, and had a higher pregnancy risk profile (31,32). The PIN3 cohort involved only one of the two primary clinics in the earlier cohorts. This representation bias might have affected our results by providing higher levels of physical activity than may be in a general pregnant population (31).
Our study was strengthened by its prospective design and large cohort, being one of the few studies of this magnitude to study women and their physical activity throughout their pregnancy. Only a few of the previously reported physical activity studies on pregnant women have used a detailed physical activity interview assessing the mode, frequency, duration, and intensity of different modes of activity (6,8,16,28), and none of them have had a large sample size with a prospective design.
Comparisons to previous literature.
In agreement with our findings, decreasing levels of overall and recreational activity during pregnancy have been reported in previous studies (9,15,20-22,25,29,34,35) in which the decline was mostly explained by the decrease in duration and intensity of recreational activity (13,21,22,25,35) as opposed to other kinds of activity. Studies on occupational physical activity have been reported in only three previous studies, in which the levels of occupational activity decreased slightly from prepregnancy to late pregnancy (7,9,20). We found stable levels of occupational physical activity among working pregnant women, yet the levels were extremely low.
We assessed separately the frequency, intensity, and duration of household-related indoor and outdoor activities and care-related activities, which have not been included in detail in any of the previous studies. Some studies have reported sustained levels in household physical activity (7,9), a shift from heavy to light domestic activity (9), or an increase in combined household and care-giving activity (33). Our findings suggest significant changes in household and care-related activity and mainly decreasing levels for light and total care giving and outdoor household activity. Moreover, our findings suggest that care giving and household physical activity contributes greatly to weekly activity and has an important role in physical activity behavior among pregnant women. Engagement in care giving and household activity was mostly of lower intensity and recreational activity of higher intensity; thus, recreational activity becomes an important contributor to energy expenditure particularly when measured by total MET hours per week.
We found few studies that explored change in activity across the different modes, from being active to inactive or from inactive to active (15,39). One study on a population-based sample of pregnant women suggested that 20% of the women started to exercise, whereas 40% became less active during pregnancy (15). Another study, using a more selected sample of white rural pregnant women, reported that 35% of pregnant women continued their exercise, 45% remained entirely inactive, 13% became inactive, and 7% initiated exercise during pregnancy (39). We reported that most of the women remained physically active in occupational, indoor household, and recreational activity.
Recommended levels of physical activity.
Previous estimates of pregnant women reaching the recommended activity levels have been carried out using recreational activity, in which the prevalence has varied from 6% to as high as 78% (12,25,26). The study that suggested that 78% of women met the recommended level of activity seems markedly higher than in other reported literature, including our findings. These discrepancies may be due to a selected population or measurement of physical activity. In this study, we used all types of activity to assess the recommended level for the CDC/ACSM definitions and exercise to assess the ACOG definition. The prevalence of sufficient activity varied between 3% and 38%, depending on the type of activity and measurement of intensity included. In our sample, using the combined CDC/ACSM moderate/vigorous intensity definition resulted in higher prevalence than the ACOG moderate-intensity exercise definition.
This study provides new information on the physical activity modes among pregnant women, particularly on household, care, and occupational activity. Further research is needed to follow women through pregnancy to postpartum to examine how physical activity levels change and to tailor a physical activity program to those women at risk for inactivity, obesity, and chronic disease.
Funding for this study was provided by the NIH/National Cancer Institute (#CA109804-01). The PIN3 study was supported by NIH/National Institute of Child Health and Human Development (#HD37584), NIH General Clinical Research Center (#RR00046), and NIH/National Institute of Diabetes and Digestive and Kidney Diseases (#OK061981-02). In addition, the Finnish Cultural Foundation and Academy of Finland are acknowledged. The PIN3 study is a joint effort of many investigators and staff members whose work is gratefully acknowledged. In addition, we thank Lisa Canada for her helpful comments on an earlier draft and the anonymous reviewers. The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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