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Perceived Exertion During Prepregnancy Physical Activity and Pre-eclampsia Risk
Carole B.Rudra; Michelle A. Williams; I-Min Lee; Raymond S. Miller; Tanya K. Sorensen 
Med Sci Sports Exerc.  2005;37(11):1836-1841.  ©2005 American College of Sports Medicine
Posted 12/16/2005

Abstract and Introduction
Abstract
Purpose: Studies have shown that absolute intensity of physical activity is associated with reduced risk of preeclampsia. This analysis was performed to examine whether relative intensity may also be related to preeclampsia risk.

Methods: Using data from a case-control study conducted from 1998 to 2002, the authors assessed the relation between perceived exertion (i.e., relative intensity) during usual recreational physical activity in the year before pregnancy and risk of preeclampsia. In a structured interview conducted during postpartum hospitalization, 244 preeclampsia cases and 470 normotensive controls provided details on their recreational physical activities in the year before pregnancy.

Results: Participants who reported feeling very strenuous to maximal exertion during usual prepregnancy physical activity were 78% less likely to have developed preeclampsia than those who reported negligible or minimal exertion (adjusted odds ratio 0.22, 95% confidence interval 0.11-0.44). A significant trend was noted of decreased preeclampsia risk with increased perceived exertion (P < 0.001). This association was independent of prepregnancy adiposity, and was also apparent among the subset of participants who did not meet physical activity guidelines in the year before pregnancy.

Conclusions: In this study, relative intensity of recreational physical activity performed during the year before pregnancy was associated with a decreased risk of preeclampsia.


Introduction
Preeclampsia is a complication of about 4% of pregnancies in the US. It is characterized by hypertension and protein in the urine. Preeclampsia can cause many problems, including low birthweight babies, still births and preterm birth. Despite years of research, there is no treatment for preeclampsia, except early delivery, which poses risk to the baby.

Recent research has show that physical activity can reduce the risk of preeclampsia. The question is “how much physical activity?” Most studies involve objective measurement of activity levels, using energy expenditure, or heart rate to measure degree of exercise. However, a new study presents evidence that exercise which a woman perceives as strenuous, regardless of actual energy spent, is correlated to a 40-80% decrease in preclampsia. And how much a woman weighs does not influence this benefit.

So what does that mean? The year prior to pregnancy is a good time to start exercising. You don’t have to run a marathon, or climb Mt Washington, but you should do some form of exercise which feels strenuous to you. Then, once you are pregnant, discuss your plans for continued exercise during pregnancy. This can help keep you and your baby healthy!

[Perceived Exertion During Prepregnancy Physical Activity and Preeclampsia Risk. Carole B.Rudra; Michelle A. Williams; I-Min Lee; Raymond S. Miller; Tanya K. Sorensen  Med Sci Sports Exerc.  2005;37(11):1836-1841]

During the year before you became pregnant, when you were exercising in your usual fashion, how would you rate your level of exertion (degree of effort)? Women selected a score based on the Borg scale of perceived exertion, which was displayed for them during the interview.[4] We categorized perceived exertion as none to weak (score 0-2), moderate (3 or 4), strenuous (5 or 6), and very strenuous to maximal (7-10).

States, is characterized by the new development of hypertension and proteinuria after midgestation.[22] The disorder is an important cause of intrauterine growth restriction, preterm birth, low birthweight, and perinatal death. Despite extensive research, currently no intervention exists other than early delivery.[22] Women and their infants could benefit from identification of modifiable behaviors that may reduce the risk of developing this disorder.

Recent and past research supports the long-hypothesized relation between physical activity and reduced risk of preeclampsia.[13,23,25] Previous analyses, including one based on the study population analyzed in this report, quantified physical activity using absolute intensity measurements.[25] These measurements include time spent or energy expended during specific physical activities, neither of which account for varying degrees of cardiovascular fitness among women.[11] For instance, general cycling, categorized as consuming eight metabolic equivalents (METs) of energy,[1] may require more physical effort from an overweight, unfit woman than a lean, fit woman.

In contrast to absolute intensity measures, relative intensity measures capture the fitness of an individual as well as the characteristics of the activity performed. One such measure, the Borg rating of perceived exertion, is used commonly in exercise stress testing.[4] It correlates well with heart rate, blood lactate, pulmonary ventilation, and oxygen consumption in men and nonpregnant women.[3]

We performed this analysis to investigate whether perceived exertion during physical activity in the year before pregnancy is associated with risk of preeclampsia. We hypothesized that perceived exertion may capture one or more aspects of maternal fitness that are not captured by absolute measures, and that these aspects may be related to preeclampsia risk. We also hypothesized that maternal adiposity may influence this relation. Specifically, adiposity could feasibly fall in the causal pathway between physical activity and preeclampsia, confound the relation, or influence the strength of the relation. Also, the relation between perceived exertion and preeclampsia may differ according to absolute intensity. The present study, with a large number of preeclampsia cases, enabled us to examine the relation independently of these two maternal characteristics.

This report is an expansion of a previously published analysis in which several absolute measures of physical activity were examined in relation to preeclampsia risk.[25] We reported a nonsignificant inverse association between energy expenditure and preeclampsia risk (comparing =36.8 MET·h·wk-1 vs none, adjusted odds ratio [OR] 0.63, 95% confidence interval [CI] 0.34-1.18). At the time of the previous manuscript submission, data on perceived exertion were not ready for analysis. We now present the energy expenditure analyses again, in a slightly different form, in this paper. We do so primarily because we examine the perceived exertion-preeclampsia relation within subgroups of women categorized by energy expenditure. Also, we anticipate that readers would like to directly compare the strengths of the perceived exertion and absolute exertion associations.

Methods
Study Setting and Population
Subjects for the present study were participants in a case-control study conducted at Swedish Medical Center and Tacoma General Hospital in Seattle and Tacoma, Washington, respectively. From 1998 to 2002, we identified English-speaking women with preeclampsia using the then-current American College of Obstetricians and Gynecologists (ACOG) guidelines to define the disorder.[2] Eligible control subjects were English-speaking women delivering on the same day as cases who had no history of pregnancy-induced hypertension or proteinuria during the study pregnancy. Of 376 eligible preeclampsia cases, 310 (82%) and 502 (53%) of 952 eligible controls elected to participate. Reasons for nonparticipation included lack of time for the interview, lack of interest in study goals, and missed appointments. In this analysis, we excluded 44 preeclampsia cases and two controls with chronic hypertension and five cases with prepregnancy diabetes mellitus because of the strong relationships between these disorders and preeclampsia. We also excluded 17 cases (5.5%) and 30 controls (6.0%) with missing information regarding physical activity in the year before pregnancy. Thus, the present analyses are based on the remaining 244 preeclampsia cases and 470 controls. Because of continued recruitment, this sample size is larger than that in the previous related paper.[25] The procedures used in this study were in agreement with the protocols approved by the institutional review boards of Swedish Medical Center and Tacoma General Hospital. All participants provided written informed consent.

Data Collection and Physical Activity Assessment
We administered structured questionnaires during participants' postpartum hospital visits to collect information on sociodemographic, medical, reproductive, and lifestyle characteristics. We also collected detailed information on habitual dietary intake during the 12 months before delivery using validated semiquantitative food frequency questionnaires (FFQ).[19]
To measure typical relative intensity of prepregnancy recreational physical activity, we asked participants, During the year before you became pregnant, when you were exercising in your usual fashion, how would you rate your level of exertion (degree of effort)? Women selected a score based on the Borg scale of perceived exertion, which was displayed for them during the interview.[4] We categorized perceived exertion as none to weak (score 0-2), moderate (3 or 4), strenuous (5 or 6), and very strenuous to maximal (7-10).

We also asked women to list the type, frequency, and duration of each recreational physical activity performed in the year before pregnancy. Based on their responses, we categorized women into two groups: those who reported regularly participating in any recreational physical activity in the year before pregnancy (active) and those who did not (inactive). We specified the energy cost of each activity, expressed as a metabolic equivalent (MET) score using a standardized classification procedure.[1] We then calculated energy expenditure during physical activity (MET·h·wk-1) using methods described previously.[25] Using a priori cutpoints, we designated inactive women as the referent group and categorized the remaining women into groups of energy expenditure: 0.1-14.9, 15.0-29.9, and =30.0 MET·h·wk-1.

Data Analysis
We examined the frequency distributions of maternal characteristics according to case-control status. We then used multivariable logistic regression to estimate the OR and 95% CI of preeclampsia according to perceived exertion and energy expenditure, separately.[10] Power and sample size estimates indicated =90% power to detect an OR of 0.50. We modeled physical activity characteristics using indicator variables. To assess confounding, we entered covariates into a logistic regression model sequentially and compared the unadjusted and adjusted regression coefficients for physical activity. The following covariates were evaluated as confounders: maternal age, race or ethnicity, prepregnancy body mass index (BMI), year of education, marital status, periconceptional fruit and vegetable consumption, smoking during pregnancy, parity, family histories of chronic hypertension and type 2 diabetes mellitus, and maternal birthweight. Final models included covariates that altered unadjusted coefficients for physical activity by =10%. Because we hypothesized that maternal adiposity could either confound the perceived intensity-preeclampsia relationship or occur in the causal pathway, we fit models with and without adjustment for prepregnancy BMI.

We also hypothesized that maternal adiposity and prepregnancy energy expenditure may influence the relation between preeclampsia and perceived exertion. Thus, we used stratified logistic regression analyses to assess these potential modifying effects. We categorized women as overweight (=25 kg·m-2) or lean (<25 kg·m-2). We dichotomized energy expenditure at 10 MET·h·wk-1. This value is approximately equivalent to the level recommended by the Centers for Disease Control and Prevention and the American College of Sports Medicine.[18] We tested for multiplicative interaction between subgroups and linear trend within subgroups defined by each of these two characteristics, separately.

We conducted a post hoc analysis, revising the definition of preeclampsia according to the criteria advocated by the National High Blood Pressure Education Program Working Group on High Blood Pressure in Pregnancy in 2000, after our study was initiated.[16] After excluding 112 proteinuric women with elevations in blood pressure who did not meet a minimum threshold of =140 mm Hg systolic or =90 mm Hg diastolic after 20 wk, 132 cases and 470 controls remained for post hoc analysis.

Results
Distributions of selected characteristics of preeclampsia cases and controls are presented in Table 1 . The women in the preeclampsia cases were more likely to be overweight, unmarried, nulliparous, and smokers during pregnancy. They were more likely to consume less than five servings of fruits and vegetables per day, to report familial hypertension history, and to have weighed less than 2500 g at birth. Thirty percent of cases and 20% of controls reported no recreational physical activity during the year before pregnancy ( Table 2 ).

Table 2 shows relative risk estimates for the relations between physical activity measures and preeclampsia. Adjustment for maternal age, race or ethnicity, parity, family history of chronic hypertension, smoking during pregnancy, and periconceptional fruit and vegetable consumption generally strengthened estimates. Although further adjustment for prepregnancy BMI slightly weakened the estimates, statistically significant inverse relations remained. After adjustment for all covariates, women reporting usual moderate exertion experienced a 46% reduction in preeclampsia risk, compared with the referent group who reported no or weak exertion (OR 0.54, 95% CI 0.26-1.12). Risk was further reduced among those reporting strenuous exertion (adjusted OR 0.33, 95% CI 0.17-0.64). Women reporting very strenuous to maximal exertion were 88% less likely to have developed preeclampsia than the referent group (adjusted OR 0.22, 95% CI 0.11-0.44). We observed a less pronounced, but still statistically significant, inverse relation between preeclampsia risk and energy expenditure during physical activity. Across increasing levels of energy expenditure, adjusted OR of preeclampsia were 0.60 (95% CI 0.36-1.02), 0.46 (95% CI 0.26-0.81), and 0.50 (95% CI 0.28-0.88), with inactive women comprising the referent group. Statistically significant inverse trends were evident across categories of both perceived exertion (P trend < 0.001) and energy expenditure (P trend = 0.01).

We found that the relation between perceived exertion and preeclampsia risk was apparent within groups of lean and overweight women, separately ( Table 3 , both P trend < 0.001). The relation did not significantly differ between these groups (P interaction = 0.33). Similarly, prepregnancy energy expenditure did not appear to have an impact on the relation between perceived intensity and preeclampsia risk ( Table 4 , P interaction = 0.53). Among women who did not meet physical activity recommendations (<10 MET·h·wk-1), those perceiving their usual activity as very strenuous to maximal were at significantly reduced preeclampsia risk, compared with women reporting negligible or weak intensity (OR 0.28, 95% CI 0.11-0.73). Among those who met or exceeded energy expenditure recommendations, the reduction in risk associated with very strenuous to maximal perceived exertion was similar (OR 0.18, 95% CI 0.03-1.11). Because only three cases and two controls who exceeded recommendations also reported negligible or weak usual exertion, however, confidence intervals surrounding the estimates in this group are wide.

After redefining preeclampsia according to the criteria advocated by the Working Group on High Blood Pressure in Pregnancy,[16] the associations between preeclampsia and physical activity measures described above did not meaningfully change (data not shown). For instance, OR across categories of increasing perceived exertion were 0.54 (95% CI 0.25-1.92), 0.33 (95% CI 0.15-0.78), and 0.22 (95% CI 0.10-0.53), with negligible to weak exertion as the referent category.


Discussion
These results suggest that the relative intensity of recreational physical activity in the year before pregnancy is strongly and inversely related to subsequent preeclampsia risk. This relation is independent of prepregnancy BMI. Most interestingly, women who do not meet physical activity recommendations in the year before pregnancy appear to benefit from engaging in activity that they perceive as strenuous.

Our findings add to data from previous studies that documented inverse associations between preeclampsia and absolute measures of physical activity intensity before and during pregnancy.[13,23,25] A 1989 case-control study documented a 43% risk reduction among Canadian women who reported any regular recreational physical activity during the first 20 wk of pregnancy (adjusted OR 0.67, 95% CI 0.46-0.96).[13] In an earlier analysis of this study population, our research group observed a 35% risk reduction among women in the western part of the state of Washington who reported engaging in any regular recreational physical activity during early pregnancy (adjusted OR 0.65, 95% CI 0.43-0.99).[25] Both studies also documented inverse trends across levels of time spent performing recreational physical activity during early pregnancy.[13,25] Most recently, in a nested case-control study conducted in Connecticut, any regular recreational leisure-time activity during pregnancy was associated with a similar risk reduction (adjusted OR 0.66, 95% CI 0.35-1.22).[23]

To our knowledge, no published studies report examining the relative intensity of physical activity in relation to preeclampsia risk. In fact, only recently have measures of relative intensity been examined in relation to chronic diseases. One study demonstrated an inverse association between perceived exertion and risk of coronary artery disease among a cohort of middle-aged men, even among those who did not meet activity recommendations.[12] The associations observed here among reproductive-aged women who did not meet recommendations before pregnancy are similarly compelling. They provide additional evidence that even low- and moderate-intensity activities may benefit members of diverse populations.

Several plausible biological mechanisms can explain why physical activity may reduce the risk of preeclampsia. Physical activity during pregnancy is associated with reduced blood pressure,[28] whereas elevated blood pressure is a primary clinical manifestation of preeclampsia.[22] Additionally, studies among healthy pregnant women indicate that physical activity is inversely related to metabolic characteristics observed among women with the disorder. These include dyslipidemia,[5,9] hyperinsulinemia,[14,26] hyperleptinemia,[6,7,17] and chronic systemic inflammation.[7,24]

Several limitations of the present study merit discussion. First, the participation rates differed between cases and controls, and were particularly low among controls. Thus, the possibility of selection bias, especially in controls, cannot be excluded. Sociodemographic characteristics of enrolled control subjects, however, were not substantially different than those of all women delivering at the two study hospitals.[8] Second, the use of retrospective questionnaires probably introduced misclassification and error in physical activity estimates. Such misclassification would result in attenuated OR if cases and controls were equally likely to misreport physical activity. We cannot exclude the possibility, however, that cases and controls differentially reported or recalled activities. Third, we collected perceived exertion measurements only in regard to activities performed during the year before pregnancy. It is unclear whether these present results can be extrapolated to activities performed during gestation. A woman's activity patterns or the exertion she feels during a particular activity can be affected by the physiologic changes that occur during a healthy pregnancy, such as increased resting heart rate and ventilatory rate, weight gain, postural changes, and increased joint laxity.[20,27] Early pregnancy nausea and fatigue can also influence activity patterns and perception of effort. Interestingly, despite these numerous pregnancy-associated physiologic changes, two studies suggest that perceived exertion during weight-supported activity appears to remain constant throughout gestation.[21,27] Finally, these results should not be extrapolated to more sedentary groups. Our study participants were more active than American women in general. Of controls, 20% reported no physical activity in the year before pregnancy. In contrast, in a 1998 national survey, 59% of American women aged 18 to 44 yr reported never engaging in activity lasting 10 min or more per week.[15] Study participants, however, were similar in physical activity habits to those female residents of the state of Washington: in a 2000 state survey, 18% reported no participation in recreational activity during the previous month (Behavioral Risk Factor Surveillance System Survey Data. Department of Health and Human Services, Centers for Disease Control and Prevention, 2000 [cited 2005 March 17]. Available from http://apps.nccd.cdc.gov/brfss/index.asp.

In summary, these data add to a growing body of literature documenting health benefits associated with exercise during and before pregnancy.[8,13,23,25] Relative intensity of physical activity may be a useful addition to other measures used in studies of the pregnancy-related benefits of physical activity.

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Table 1. Distributions of preeclampsia cases and control subjects according to selected characteristics: Washington State, 1998-2002.

Table 2. Unadjusted and adjusted odds ratios (OR) and 95% confidence intervals (CI) for preeclampsia according to recreational physical activity performed during the year before pregnancy: Washington State, 1998-2002.

Table 3. Adjusted odds ratios (OR) and 95% confidence intervals (CI) for preeclampsia according to perceived exertion during prepregnancy recreational physical activity, in subgroups defined by prepregnancy body mass index (BMI).

Table 4. Adjusted odds ratios (OR) and 95% confidence intervals (CI) for preeclampsia according to perceived exertion during prepregnancy recreational physical activity, in subgroups defined by prepregnancy energy expenditure.


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