Abstract
Previous case-control studies suggest that recreational physical activity protects against preeclampsia. Using a prospective design, the authors estimated the risk of preeclampsia for pregnant women according to level of physical activity, taking other variables that influence risk into consideration. The data set comprised 59,573 pregnancies from the Norwegian Mother and Child Cohort Study (1999–2006). Information on physical activity and other exposures was extracted from questionnaire responses given in pregnancy weeks 14–22, whereas diagnosis of preeclampsia was retrieved from the Medical Birth Registry of Norway. Estimation and confounder control was performed with multiple logistic regression. About 24% of pregnant women reported no physical activity, and 7% reported more than 25 such activities per month. The adjusted odds ratio was 0.79 (95% confidence interval: 0.65, 0.96) for preeclampsia when comparing women who exercised 25 times or more per month with inactive women. The association appeared strongest among women whose body mass index was less than 25 kg/m2 and was absent among women whose body mass index was higher than 30 kg/m2. These results suggest that the preventive effect of recreational physical activity during pregnancy may be more limited than has been shown in case-control studies and may apply to nonobese women only.
Keywords: body mass index, cohort studies, exercise, pre-eclampsia, pregnancy
The causes of preeclampsia are unknown, and no sound advice for primary prevention can be given to pregnant women (1). It has been hypothesized that regular physical activity during pregnancy may stimulate placental growth, reduce oxidative stress, and reverse maternal endothelial dysfunction (2). A review of the literature on this topic finds that leisure-time physical activity has a clear protective effect on the development of preeclampsia (3). On the basis of postpartum interviews with 244 women with preeclampsia and 470 controls in Seattle, Washington, Rudra et al. (4), expanding a previous report from the same study population (5), reported that the relative intensity of recreational physical activity in the year before pregnancy, measured by the Borg scale of perceived exertion, was associated with a reduction in the risk of preeclampsia (odds ratio = 0.22, 95% confidence interval: 0.11, 0.44 for the highest exertion level vs. inactivity). A case-control study from Canada reported an odds ratio of 0.67 (95% confidence interval: 0.46, 0.96) for any recreational physical activity in the first 20 weeks of pregnancy (6). A cohort study from Connecticut, examining 44 cases with preeclampsia among 2,638 pregnant women, suggested that physical activity both at work and in leisure time during pregnancy reduced the risk, although not statistically significant, whereas leisure-time physical activity prior to pregnancy had no effect (7).
Our aim was to study the possible protective effect of recreational physical activity during pregnancy on preeclampsia by analyzing a well-characterized, large cohort of Norwegian women.
MATERIALS AND METHODS
The present study is a subproject of the Norwegian Mother and Child Cohort Study (MoBa) (8). In brief, MoBa is a nationwide pregnancy cohort that, in 1999–2008, aims to include 100,000 pregnancies and to follow parents and children to understand causes of diseases. Pregnant women are recruited for the study through a postal invitation after they have signed up for the routine ultrasound examination at their local hospital. The participation rate is 43.8%, and 50 of 52 hospitals with maternity wards take part in the recruitment (8).
Participants receive a questionnaire asking about previous diseases, lifestyle habits, medications, and present health status. The median pregnancy week when the questionnaire is completed is week 17, with the 5th and 95th percentiles being weeks 14 and 22, respectively. The questionnaire is completed by 94.9% of participants (8). We used a quality-assured questionnaire data file released for research in 2007 with information on 67,355 pregnancies. We excluded from consideration 2,555 pregnancies in which the women had responded to a first version of the questionnaire that asked only for weekly (and not monthly) frequency of physical activity. In addition, data on physical activity during pregnancy were missing for 2,447 women who responded to the second version of the questionnaire, leaving 62,353 eligible pregnancies for further study. The record in the Medical Birth Registry of Norway (9) from the present pregnancy is included as part of the data set. The questionnaire data file was successfully linked to 59,573 singleton births registered in the Medical Birth Registry of Norway, and this sample constituted the population under study. MoBa has been approved by the Regional Committee for Ethics in Medical Research and the Data Inspectorate.
The main outcome variable was preeclampsia as registered in the Medical Birth Registry of Norway. Information provided to the registry is based on forms completed by midwives after birth. The form has 5 checkoff boxes relevant to preeclampsia: hemolysis, elevated liver enzymes, and low platelet count (HELLP); eclampsia; and early (diagnosed before 34 weeks), mild, and severe preeclampsia. In addition, the form has an open field for text that may include information leading to 1 of these diagnoses or that leads to the diagnosis of unspecified preeclampsia. For the present study, the diagnosis of preeclampsia was given if any of the above-mentioned diagnoses were present. Severe preeclampsia was used if 1 or more of the following diagnoses were given: severe or early preeclampsia; hemolysis, elevated liver enzymes, and low platelet count; or eclampsia, whereas mild or unspecified preeclampsia was used when these diagnoses were given alone. The diagnostic criteria for preeclampsia in Norway, according to guidelines issued by the Society for Gynecology, are blood pressure of ≥140/90 mm Hg after 20 weeks of gestation combined with proteinuria ≥+1 dipstick on at least 2 occasions. A diagnosis of severe preeclampsia requires a blood pressure of >160/110 mm Hg and ≥3 g of protein in a 24-hour urine sample (10). Maternal age at delivery was also retrieved from the Medical Birth Registry of Norway record.
In the questionnaire, the participants were asked how often they performed the following 14 activities during pregnancy: brisk walking, running, bicycling, attendance at training studios, prenatal aerobics classes, low-impact aerobics classes, high-impact aerobics classes, dancing, skiing, team sports, swimming, walking, horseback riding, or other. We decided to combine the first 11 activities into an overall score. For each activity, the respondents could choose between the following categories: never (score: 0), 1–3 times per month (monthly frequency score: 2), once per week (score: 4), twice per week (score: 8), or 3 or more times per week. For respondents who reported 3 or more times per week, we inserted the median number (among responders who reported at least 3 times weekly) from the distribution of responses to an open question on the weekly frequency of the same physical activities in the above-mentioned first version of the questionnaire. These median numbers were 3 (all other activities), 4 (skiing, dancing, running), or 5 (brisk walking), corresponding to 12, 16, or 20 times per month. The monthly frequency scores were then summed across all 11 activities.
From the questionnaire we also included parity (0, ≥1) as well as prepregnancy height and weight, which were used to calculate body mass index (BMI: weight (kg)/height (m)2). BMI was analyzed as a continuous or categorical variable (<18.5, 18.5–24.9, 25.0–29.9, 30.0–34.9, ≥35.0). Height was categorized as <165, 165–168, 169–172, and ≥173 cm. Smoking during pregnancy (nonsmoker, occasional smoker, daily smoker), educational attainment (<12, 12, 13–16, ≥17 years), as well as the response to the statement “my work is physically strenuous” (response categories: “correct,” “somewhat correct,” “not quite correct,” “not correct”), which was posed to women who reported being in paid jobs, were also included as potentially confounding variables. For these confounders, a missing response category was included in the analyses. The questionnaire can be examined at the following website: www.fhi.no/morogbarn. The relative risks of preeclampsia according to physical activity were approximated by calculating odds ratios. Adjustment for confounding was obtained by estimating adjusted odds ratios in multiple logistic regression analyses (11).
RESULTS
The cumulative incidence of preeclampsia (any diagnosis) was 3.9% (2,315/59,573). Eclampsia was noted in 31 pregnancies and the hemolysis, elevated liver enzymes, and low platelet count syndrome in 104, whereas early preeclampsia was diagnosed in 237 women. The incidence of severe preeclampsia was 1.1% (n = 655), the incidence of mild preeclampsia was 2.3% (n = 1,356), and the incidence of unspecified preeclampsia was 0.5% (n = 304).
Table 1 gives the distribution of recreational physical activity per month by maternal characteristics and shows that the highest levels were found among primiparous women, lean women, women with high educational attainment, and nonsmokers. Table 2 shows that the incidence of preeclampsia was lower (3.2%) among highly active women when compared with sedentary women (4.1%). Both smoking and parity are confounders that may mask the effect of physical activity on preeclampsia. In model 1 (Table 2), the odds ratios were reduced compared with the unadjusted odds ratios when these two variables were included in the logistic regression analysis. However, when prepregnancy BMI was included in addition, the odds ratios increased to close to the unadjusted levels (Table 2, model 2). Introduction of other possible confounders in the regression analysis had little effect (Table 2, model 3). When all confounders were included, the odds ratio for preeclampsia was 0.79 (95% confidence interval: 0.65, 0.96) when women with more than 25 recreational physical activities per month were compared with women who reported no such activities. When all exercising women were contrasted to inactive women, the odds ratio for preeclampsia was 0.94 (95% confidence interval: 0.85, 1.04).
Table 1.
Percentage Distribution of Recreational Physical Activitya According to Characteristics of Women in the Norwegian Mother and Child Cohort Study, 1999–2006
| No. | No. of Activities per Month |
|||||
| 0 | 1–5 | 6–12 | 13–24 | ≥25 | ||
| Maternal age at delivery, years | ||||||
| <20 | 613 | 28.5 | 26.3 | 21.0 | 17.0 | 7.2 |
| 20–24 | 6,184 | 26.6 | 27.6 | 23.5 | 16.0 | 6.4 |
| 25–29 | 20,008 | 22.5 | 27.3 | 25.5 | 17.1 | 7.6 |
| 30–34 | 22,908 | 23.2 | 27.3 | 25.8 | 16.8 | 6.9 |
| ≥35 | 9,860 | 24.7 | 27.4 | 25.8 | 16.0 | 6.0 |
| Prepregnancy body mass index, kg/m2 | ||||||
| <18.5 | 1,804 | 26.8 | 25.3 | 21.7 | 18.0 | 8.1 |
| 18.5–24.9 | 37,547 | 21.4 | 25.9 | 26.3 | 18.1 | 8.2 |
| 25.0–29.9 | 12,853 | 25.8 | 30.0 | 25.2 | 14.2 | 4.8 |
| 30.0–34.9 | 4,176 | 30.1 | 31.2 | 22.5 | 12.9 | 3.3 |
| ≥35 | 1,543 | 32.5 | 34.3 | 19.7 | 10.6 | 2.9 |
| No response | 1,650 | 26.7 | 24.3 | 24.0 | 17.9 | 7.1 |
| Smoking | ||||||
| Nonsmoker | 53,616 | 22.4 | 27.2 | 26.0 | 17.1 | 7.3 |
| Occasional smoker | 1,811 | 26.1 | 27.4 | 24.4 | 15.8 | 6.4 |
| Daily smoker | 3,709 | 39.3 | 28.5 | 18.1 | 11.4 | 2.8 |
| No response | 437 | 31.1 | 24.7 | 20.8 | 17.4 | 5.9 |
| Height, cm | ||||||
| <165 | 16,013 | 26.8 | 27.7 | 24.1 | 15.3 | 6.1 |
| 165–168 | 15,163 | 22.7 | 27.9 | 25.6 | 16.8 | 7.0 |
| 169–172 | 14,271 | 22.2 | 27.2 | 26.3 | 17.1 | 7.2 |
| ≥173 | 13,428 | 22.0 | 26.6 | 25.9 | 17.7 | 7.8 |
| No response | 698 | 27.8 | 22.6 | 22.9 | 18.6 | 8.0 |
| Parity | ||||||
| 0 | 27,084 | 20.3 | 24.5 | 25.8 | 19.9 | 9.5 |
| ≥1 | 32,489 | 26.3 | 29.7 | 25.1 | 14.1 | 4.8 |
| Educational attainment, years | ||||||
| <12 | 12,196 | 34.4 | 28.2 | 20.8 | 12.8 | 3.8 |
| 12 | 7,775 | 28.2 | 29.2 | 22.4 | 15.0 | 5.2 |
| 13–16 | 24,876 | 20.6 | 27.9 | 27.3 | 17.1 | 7.1 |
| ≥17 | 13,212 | 15.9 | 24.5 | 28.3 | 20.5 | 10.8 |
| No response | 1,514 | 29.1 | 26.2 | 22.4 | 17.2 | 5.2 |
| Physically demanding job | ||||||
| No job | 4,632 | 34.3 | 26.1 | 19.5 | 14.9 | 5.3 |
| Not correct | 23,638 | 21.5 | 27.5 | 27.2 | 16.7 | 7.1 |
| Not quite correct | 12,945 | 21.2 | 27.6 | 26.9 | 16.9 | 7.5 |
| Somewhat correct | 9,827 | 22.7 | 27.9 | 25.1 | 17.2 | 7.1 |
| Correct | 7,332 | 27.8 | 26.2 | 22.2 | 17.0 | 6.8 |
| No response | 1,199 | 30.9 | 27.1 | 21.1 | 15.8 | 5.0 |
| Year of childbirth | ||||||
| 1999–2001 | 3,019 | 25.5 | 27.4 | 26.5 | 15.1 | 5.5 |
| 2002 | 7,699 | 24.3 | 28.8 | 25.6 | 14.8 | 6.5 |
| 2003 | 11,216 | 24.6 | 27.7 | 25.5 | 15.9 | 6.3 |
| 2004 | 11,952 | 24.1 | 27.1 | 24.9 | 17.1 | 6.8 |
| 2005 | 13,968 | 23.0 | 27.1 | 25.1 | 17.5 | 7.3 |
| 2006 | 11,719 | 21.8 | 26.4 | 25.9 | 17.8 | 8.1 |
| Total | 59,573 | 23.6 | 27.3 | 25.4 | 16.7 | 7.0 |
Number of activities per month.
Table 2.
Crude and Adjusted Odds Ratios for Preeclampsia According to Recreational Physical Activity During Pregnancy, Norwegian Mother and Child Cohort Study, 1999–2006
| No. of Activities per Month | No. of Pregnancies | No. of Casesa | Incidence | Crude OR | Model 1b |
Model 2c |
Model 3d |
|||
| OR | 95% CI | OR | 95% CI | OR | 95% CI | |||||
| 0 | 14,054 | 583 | 4.1 | 1.0 | 1.0 | 1.0 | 1.0 | |||
| 1–5 | 16,272 | 642 | 3.9 | 0.95 | 0.92 | 0.82, 1.04 | 0.94 | 0.84, 1.05 | 0.96 | 0.85, 1.07 |
| 6–12 | 15,147 | 589 | 3.9 | 0.94 | 0.86 | 0.77, 0.97 | 0.94 | 0.83, 1.05 | 0.96 | 0.85, 1.08 |
| 13–24 | 9,950 | 367 | 3.7 | 0.89 | 0.77 | 0.67, 0.88 | 0.87 | 0.76, 0.99 | 0.89 | 0.78, 1.03 |
| ≥25 | 4,150 | 134 | 3.2 | 0.77 | 0.62 | 0.51, 0.76 | 0.76 | 0.62, 0.92 | 0.79 | 0.65, 0.96 |
Abbreviations: CI, confidence interval; OR, odds ratio.
All cases of preeclampsia, including the hemolysis, elevated liver enzymes, and low platelet count (HELLP) syndrome and eclampsia.
Adjusted for smoking and parity.
Adjusted for smoking, parity, and prepregnancy body mass index.
Adjusted for smoking, parity, prepregnancy body mass index, educational attainment, maternal age at delivery, year of childbirth, height, and physically demanding job.
In Table 3, the effects of physical activity are examined separately for severe, mild, and unspecified preeclampsia. In general, the odds ratios were not statistically significantly different from 1, although there was a tendency toward reduced odds ratios for the highest activity levels. There were no indications of effect modification when analyses were performed within strata of parity, smoking, or work participation. Table 4 shows a clear effect of physical activity for subjects whose BMI was lower than 25 and that all estimates were above 1 for physically active women whose BMI was higher than 30.
Table 3.
Crude and Adjusted Odds Ratios for Severe, Mild, and Unspecified Preeclampsia According to Recreational Physical Activity, Norwegian Mother and Child Cohort Study, 1999–2006
| No. of Activities per Month | No. of Pregnancies | Severe Preeclampsiaa |
Mild Preeclampsia |
Unspecified Preeclampsia |
|||||||||
| No. | % | Adjusted ORb | 95% CI | No. | % | Adjusted ORb | 95% CI | No. | % | Adjusted ORb | 95% CI | ||
| 0 | 14,054 | 153 | 1.1 | 1.0 | 350 | 2.5 | 1.0 | 80 | 0.6 | 1.0 | |||
| 1–5 | 16,272 | 166 | 1.0 | 0.94 | 0.75, 1.17 | 389 | 2.4 | 0.97 | 0.83, 1.12 | 87 | 0.5 | 0.95 | 0.70, 1.29 |
| 6–12 | 15,147 | 176 | 1.2 | 1.07 | 0.85, 1.33 | 337 | 2.2 | 0.93 | 0.79, 1.08 | 76 | 0.5 | 0.91 | 0.66, 1.25 |
| 13–24 | 9,950 | 123 | 1.2 | 1.10 | 0.86, 1.40 | 198 | 2.0 | 0.82 | 0.69, 0.99 | 46 | 0.5 | 0.81 | 0.56, 1.18 |
| ≥25 | 4,150 | 37 | 0.9 | 0.78 | 0.54, 1.13 | 82 | 2.0 | 0.84 | 0.66, 1.08 | 15 | 0.4 | 0.64 | 0.36, 1.12 |
Abbreviations: CI, confidence interval; OR, odds ratio.
Includes cases from the Medical Birth Registry of Norway noted as severe and, additionally, preeclampsia occurring before 37 weeks; the hemolysis, elevated liver enzymes, and low platelet count (HELLP) syndrome; and eclampsia.
Adjusted for smoking, parity, prepregnancy body mass index, educational attainment, maternal age at delivery, parity, year of childbirth, height. and physically demanding job.
Table 4.
Adjusted Odds Ratios for Preeclampsiaa According to Recreational Physical Activity and Categories of Prepregnancy BMI, Norwegian Mother and Child Cohort Study, 1999–2006
| No. of Activities per Month | Prepregnancy BMI <25 |
BMI 25.0–29.9 |
BMI ≥30 |
||||||||||||
| No. of Pregnancies | No. of Cases | % | Adjusted ORb | 95% CI | No. of Pregnancies | No. of Cases | % | Adjusted ORb | 95% CI | No. of Pregnancies | No. of Cases | % | Adjusted ORb | 95% CI | |
| 0 | 8,535 | 260 | 3.0 | 1.0 | 3,318 | 175 | 5.3 | 1.0 | 1,760 | 128 | 7.2 | 1.0 | |||
| 1–5 | 10,190 | 272 | 2.7 | 0.85 | 0.71, 1.01 | 3,850 | 207 | 5.4 | 1.00 | 0.81, 1.23 | 1,831 | 150 | 8.2 | 1.20 | 0.93, 1.54 |
| 6–12 | 10,263 | 273 | 2.7 | 0.80 | 0.67, 0.96 | 3,244 | 170 | 5.2 | 0.94 | 0.75, 1.17 | 1,244 | 121 | 9.7 | 1.47 | 1.12, 1.91 |
| 13–24 | 7,123 | 191 | 2.7 | 0.77 | 0.63, 0.93 | 1,830 | 107 | 5.8 | 0.99 | 0.77, 1.27 | 702 | 60 | 8.5 | 1.21 | 0.87, 1.67 |
| ≥25 | 3,240 | 83 | 2.6 | 0.69 | 0.53, 0.89 | 611 | 26 | 4.3 | 0.68 | 0.44, 1.04 | 182 | 19 | 10.4 | 1.39 | 0.83, 2.32 |
Abbreviations: BMI, body mass index (weight (km)/height (m)2); CI, confidence interval; OR, odds ratio.
Includes severe, mild, and unspecified cases of preeclampsia.
Adjusted for smoking, parity, prepregnancy BMI (continuous measure), educational attainment, maternal age at delivery, parity, year of childbirth, height, and physically demanding job.
DISCUSSION
These results suggest that recreational physical activity during pregnancy reduces the risk of preeclampsia. The women who reported frequent physical activity had a 20% reduction in risk. Stronger effects (30%–80% reduction in risk) have been found in case-control studies (4–6). There may be several explanations for this discrepancy. In contrast to the case-control studies, MoBa was not designed to specifically study physical activity and preeclampsia, perhaps reducing the potential for selection related to the study question. The exposure measures in the case-control studies were based on recall, which may be influenced by the outcome. On the other hand, a limitation of MoBa is the precision and validity of the exposure measurements. We did not know the intensity of the physical activities reported by the pregnant women, and misclassification is likely. It is reasonable to assume that this information error would bias the odds ratio estimates toward the null value. However, differential information bias is unlikely in MoBa since physical activity and confounders were measured prior to diagnosis of the endpoint, which is taken from an independent source, the birth registry.
Better resolution of the relation between recreational physical activity and preeclampsia could come from randomized, controlled trials specifically designed to address this research question. A Cochrane review (12) has shown that no conclusions can be drawn from the existing 2 small trials (13, 14), which were designed mainly for other purposes. In combination, the intervention group in these 2 studies included 23 women and the control group 22 women. One case of preeclampsia occurred in the control group, giving a relative risk of 0.31 (95% confidence interval: 0.01, 7.09).
We could not find any protective effect of recreational physical activity for women with a prepregnancy BMI above 30. However, this analysis was not performed as a consequence of a prior hypothesis, and this may be a chance finding. The question should be examined by others before any advice to obese women is given. Rudra et al. (4), in contrast, found a relatively strong protective effect of physical activity among women whose BMI was above 25. However, in that analysis, residual confounding may have been present because adjustment for the BMI levels within the 2 strata (below and above 25) was not performed.
Several mechanisms for a protective effect of recreational physical activity on the risk of preeclampsia may exist. In nonpregnant women, exercise is known to reduce blood pressure (15) and triglyceride levels (16). Dyslipidemia, inflammation, and oxidative stress are features of preeclampsia. It has been shown that regular weight-bearing exercise influences the level of plasma tumor necrosis factor-α during pregnancy (17). Preeclampsia is said to share the same pathologies as cardiovascular disease, with endothelial dysfunction as a common denominator (18), and the 2 conditions share some risk factors such as obesity and diabetes; however, whereas smoking increases the risk of cardiovascular disease, it paradoxically reduces the risk of preeclampsia independently of BMI level (19).
Some of the pathophysiological findings in preeclampsia may be secondary to early placental events. Successful placentation involves adequate invasion of the extravillous cytotrophoblasts into the maternal uterine spiral arteries, securing wide, low-resistance vascular channels providing the developing fetus with maximum blood flow. Bergmann et al. (20) has shown that running throughout pregnancy has a favorable effect on placental villous vascular volume. Further studies of obese and lean women are needed to understand the mechanisms and timing of the effects of exercise. More detailed studies in MoBa, utilizing the available biologic material obtained during pregnancy and after birth (8), may assist in understanding the mechanisms behind the effects of physical activity and other factors on the occurrence of hypertensive disorders in pregnancy.
In summary, we found that recreational physical activity may have a moderately protective effect on the risk of preeclampsia, at least for women with a BMI of less than 25.
Acknowledgments
Author affiliations: Division of Epidemiology, Norwegian Institute of Public Health, Oslo, Norway (Per Magnus, Lill Trogstad, Katrine Owe, Wenche Nystad); Department of Obstetrics and Gynecology, Ullevaal University Hospital, Oslo, Norway (Lill Trogstad); Department of Sports Medicine, Norwegian School of Sport Sciences, Oslo, Norway (Katrine Owe); Maternal Nutrition Group, Department of Epidemiology Research, Statens Serum Institut, Copenhagen, Denmark (Sjurdur F. Olsen); and Department of Nutrition, Harvard School of Public Health, Boston, Massachusetts (Sjurdur F. Olsen).
The Norwegian Ministry of Health, The Norwegian Research Council (contract 151918/S10), National Institute of Environmental Health Sciences (contract N01ES75558), National Institute of Neurological Diseases and Stroke (contract 1 UO1 NS 047537). Parallel analyses have been undertaken from another large prospective database, the Danish National Birth Cohort (21), and The 6th Research Framework of the European Union (EARNEST) financially supported the interaction between the 2 teams.
Conflict of interest: none declared.
Glossary
Abbreviations
- BMI
body mass index
- MoBa
Norwegian Mother and Child Cohort Study
References
- 1.Sibai BM. Prevention of preeclampsia: a big disappointment. Am J Obstet Gynecol. 1998;179(5):1275–1278. doi: 10.1016/s0002-9378(98)70146-2. [DOI] [PubMed] [Google Scholar]
- 2.Weissgerber TL, Wolfe LA, Davies GAL. The role of regular physical activity in preeclampsia prevention. Med Sci Sports Exerc. 2004;36(12):2024–2031. doi: 10.1249/01.mss.0000147627.35139.dc. [DOI] [PubMed] [Google Scholar]
- 3.Hegaard HK, Pedersen BK, Bruun Nielsen B, et al. Leisure time physical activity during pregnancy and impact on gestational diabetes mellitus, pre-eclampsia, preterm delivery and birth weight: a review. Acta Obstet Gynecol Scand. 2007;86(11):1290–1296. doi: 10.1080/00016340701647341. [DOI] [PubMed] [Google Scholar]
- 4.Rudra CB, Williams MA, Lee IM, et al. Perceived exertion during prepregnancy activity and preeclampsia risk. Med Sci Sports Exerc. 2005;37(11):1836–1841. doi: 10.1249/01.mss.0000175862.41620.41. [DOI] [PubMed] [Google Scholar]
- 5.Sorensen TK, Williams MA, Lee IM, et al. Recreational physical activity during pregnancy and risk of preeclampsia. Hypertension. 2003;41(6):1273–1280. doi: 10.1161/01.HYP.0000072270.82815.91. [DOI] [PubMed] [Google Scholar]
- 6.Marcoux S, Brisson J, Fabia J. The effect of leisure time physical activity on the risk of pre-eclampsia and gestational hypertension. J Epidemiol Community Health. 1989;43(2):147–152. doi: 10.1136/jech.43.2.147. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 7.Saftlas AF, Logsden-Sackett N, Wang W, et al. Work, leisure-time physical activity, and risk of preeclampsia and gestational hypertension. Am J Epidemiol. 2004;160(8):758–765. doi: 10.1093/aje/kwh277. [DOI] [PubMed] [Google Scholar]
- 8.Magnus P, Irgens LM, Haug K, et al. Cohort profile: the Norwegian Mother and Child Cohort Study (MoBa) Int J Epidemiol. 2006;35(5):1146–1150. doi: 10.1093/ije/dyl170. [DOI] [PubMed] [Google Scholar]
- 9.Irgens LM. The Medical Birth Registry of Norway. Epidemiological research and surveillance throughout 30 years. Acta Obstet Gynecol Scand. 2000;79(6):435–439. [PubMed] [Google Scholar]
- 10.Dalaker K, Berle EJ. Clinical Guidelines in Obstetrics 1999. Oslo, Norway: The Norwegian Society of Obstetrics and Gynecology, Norwegian Medical Association; 1999. [Google Scholar]
- 11.SPSS. version 14, software. Chicago, IL: SPSS Inc; 2005. [Google Scholar]
- 12.Meher S, Duley L. Exercise or other physical activity for preventing pre-eclampsia and its complications [electronic article] Cochrane Database Syst Rev. 2006 Apr 19;(2):CD005942. doi: 10.1002/14651858.CD005942. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 13.Avery MD, Leon AS, Kopher RA. Effects for a partially home-based exercise program for women with gestational diabetes. Obstet Gynecol. 1997;89(1):10–15. doi: 10.1016/s0029-7844(97)84256-1. [DOI] [PubMed] [Google Scholar]
- 14.Yeo S, Steele N, Chang MC, et al. Effect of exercise on blood pressure in pregnant women with a high risk of gestational hypertensive disorders. J Reprod Med. 2000;45(4):293–298. [PubMed] [Google Scholar]
- 15.Kelly GA. Aerobic exercise and resting blood pressure among women: a meta-analysis. Prev Med. 1999;28(3):264–275. doi: 10.1006/pmed.1998.0417. [DOI] [PubMed] [Google Scholar]
- 16.Durstine JL, Grandjean PW, Davis PG, et al. Blood lipids and lipoprotein adaptations to exercise: a quantitative analysis. Sports Med. 2001;31(15):1033–1062. doi: 10.2165/00007256-200131150-00002. [DOI] [PubMed] [Google Scholar]
- 17.Clapp JF, III, Kiess W. Effects of pregnancy and exercise on concentrations of the metabolic markers tumor necrosis factor α and leptin. Am J Obstet Gynecol. 2000;182(2):300–306. doi: 10.1016/s0002-9378(00)70215-8. [DOI] [PubMed] [Google Scholar]
- 18.Chambers JC, Fusi L, Malik IS, et al. Association of maternal endothelial dysfunction with preeclampsia. JAMA. 2001;285(12):1607–1612. doi: 10.1001/jama.285.12.1607. [DOI] [PubMed] [Google Scholar]
- 19.Stone CD, Diallo O, Shyken J, et al. The combined effect of maternal smoking and obesity on the risk of preeclampsia. J Perinat Med. 2007;35(1):28–31. doi: 10.1515/JPM.2007.003. [DOI] [PubMed] [Google Scholar]
- 20.Bergmann A, Zygmunt M, Clapp JF., III Running throughout pregnancy: effect on placental villous vascular volume and cell proliferation. Placenta. 2004;25(8–9):694–698. doi: 10.1016/j.placenta.2004.02.005. [DOI] [PubMed] [Google Scholar]
- 21.Østerdal ML, Strøm M, Klemmensen AK, et al. Does leisure time physical activity protect against preeclampsia? Prospective cohort in Danish women. BJOG. doi: 10.1111/j.1471-0528.2008.02001.x. In press. [DOI] [PubMed] [Google Scholar]