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Is Swimming During Pregnancy a Safe Exercise?

Juhl, Mettea; Kogevinas, Manolisb,c,d,e; Andersen, Per Kraghf; Andersen, Anne-Marie Nybog; Olsen, Jørnh

doi: 10.1097/EDE.0b013e3181cb6267
Pregnancy: Original Article
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Background: Exercise in pregnancy is recommended in many countries, and swimming is considered by many to be an ideal activity for pregnant women. Disinfection by-products in swimming pool water may, however, be associated with adverse effects on various reproductive outcomes. We examined the association between swimming in pregnancy and preterm and postterm birth, fetal growth measures, small-for-gestational-age, and congenital malformations.

Methods: We used self-reported exercise data (swimming, bicycling, or no exercise) that were prospectively collected twice during pregnancy for 74,486 singleton pregnancies. Recruitment to The Danish National Birth Cohort took place 1996–2002. Using Cox, linear and logistic regression analyses, depending on the outcome, we compared swimmers with physically inactive pregnant women; to separate a possible swimming effect from an effect of exercise, bicyclists were included as an additional comparison group.

Results: Risk estimates were similar for swimmers and bicyclists, including those who swam throughout pregnancy and those who swam more than 1.5 hours per week. Compared with nonexercisers, women who swam in early/mid-pregnancy had a slightly reduced risk of giving birth preterm (hazard ratio = 0.80 [95% confidence interval = 0.72–0.88]) or giving birth to a child with congenital malformations (odds ratio = 0.89 [0.80–0.98]).

Conclusions: These data do not indicate that swimming in pool water is associated with adverse reproductive outcomes.

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From the aNational Institute of Public Health, University of Southern Denmark, Denmark; bCentre for Research in Environmental Epidemiology (CREAL), Barcelona, Spain; cMunicipal Institute of Medical Research (IMIM-Hospital del Mar), Barcelona, Spain; dCIBER Epidemiologia y Salud Pública (CIBERESP), Barcelona, Spain; eMedical School, University of Crete, Heraklion, Greece; fInstitute of Public Health, Department of Biostatistics, University of Copenhagen, Denmark; gDepartment of Epidemiology, Faculty of Health Sciences, University of Southern Denmark, Odense, Denmark; and hDepartment of Epidemiology, UCLA School of Public Health, Los Angeles, CA.

Submitted 5 May 2009; accepted 26 August 2009; posted 28 January 2010.

Supported by the Danish Medical Research Council, the Augustinus Foundation, The Danish Midwifery Association, the Danish Graduate School in Public Health Sciences, and The Danish National Board of Health.

Supplemental digital content is available through direct URL citations in the HTML and PDF versions of this article (www.epidem.com).

Correspondence: Mette Juhl, National Institute of Public Health, 5, Øster Farimagsgade, DK-1399 Copenhagen K, Denmark. E-mail: metjuhl@gmail.com.

Some disinfection by-products in drinking water may be fetotoxic and thus increase the risk of adverse reproductive outcomes among highly exposed pregnant women. Endpoints such as time-to-pregnancy,1 preterm birth,2 low birth weight or small-for-gestational-age (SGA),3–14 spontaneous abortions,15–22 or birth defects3,15,23–30 have been studied, but the empirical evidence for any of these potential hazards is inconclusive and potential biologic mechanisms are poorly understood.10 Chemical disinfection processes in swimming pools result in the formation of disinfection by-products through the reaction of chlorine with organic matter.31 Swimming pool water contains natural organic matter from the tap water itself and also from bathers' sweat, urine, skin particles, hair, microorganisms, cosmetics, and other personal care products.32 The specific types and levels of disinfection by-products depend on numerous factors, including the type and amount of disinfectant used, characteristics of the swimming pool and pool water, and swimmer hygiene.31 Exposure to disinfection by-products from swimming pools may occur through ingestion, inhalation, or dermal absorption when swimming.33–40 For several by-products including the most common trihalomethanes, inhalation and dermal absorption contribute more to the total uptake than does ingestion.41 Only one study has investigated the association of swimming in pools with birth weight; no association was apparent.42 Drinking water in Denmark is rarely chlorinated, which would make it easier to isolate possible health effects of chlorinated swimming pool water. Further, swimming in indoor pools is a recommended and common activity during pregnancy.43,44

We examined the association between swimming during pregnancy and several birth outcomes (preterm and postterm birth; fetal growth measures, including birth weight and SGA; and congenital malformations) in a large cohort of pregnant women in Denmark. We compared swimmers and bicyclists with physically inactive pregnant women. Bicyclists were included as a comparison group in the attempt to separate a possible swimming effect from that of exercise per se. Exercise, in general, has been evaluated in previous publications.43,45,46

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METHODS

Subjects

The Danish National Birth Cohort is a nationwide population-based cohort with prospectively collected data from pregnant women and their offspring. The intention during recruitment was to invite as many eligible women in Denmark as possible until the goal of 100,000 pregnancies had been reached; inclusion criteria were residence in Denmark, no plans for an induced abortion, and fluency in Danish sufficient to participate in 4 telephone interviews during and after pregnancy. Recruitment took place between 1996 and 2002, using general practitioners to present the consent form together with written information at the first antenatal visit in early pregnancy. The initial data collection included telephone interviews, questionnaires, and blood samples. The 2 pregnancy interviews used in this study were scheduled to take place in pregnancy weeks 12–16 and 30. The median gestational age for the first pregnancy interview was 114 days (10th percentile, 84 days; 90th percentile, 160 days), corresponding to 16.3 completed weeks. For the second interview, the median gestational age was 218 days (10th percentile, 195 days; 90th percentile, 249 days), corresponding to 31.1 completed weeks. About half of the general practitioners agreed to take part in the recruitment process. We estimate that about 60% of the invited women accepted the invitation. More details about the cohort have been presented elsewhere.47

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Study Population(s)

For this analysis, we included all pregnancies with available data from the first pregnancy interview (n = 90,165). This excludes women who had an induced or spontaneous abortion before the time of the first interview, or who could not be contacted by phone for this interview. We also excluded 1965 multiple pregnancies. We included only pregnancies for which the mother had reported swimming, bicycling, or no exercise. Main analyses were based on swimming data from the first pregnancy interview. The Figure describes the selection of women to this study. Specific exclusions were made for each of the 4 endpoints studied, and this resulted in minor variations in sample size (Fig.). To be included in additional analyses of swimming throughout pregnancy, the women needed to have reported swimming, bicycling, or no exercise in both interviews. Because many women changed their exercise habits from the first to the second interview, there is a difference of about 25,000 pregnancies between the 2 analyses. Altogether, 48,781 pregnancies were included in this study. (See eTable; http://links.lww.com/EDE/A365).

FIGURE.

FIGURE.

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Data on Swimming

We used self-reported data from the first and the second pregnancy interview on engagement in physical exercise (yes/no), type of exercise, and duration of each exercise session. We generated 3 exposure categories: (1) any reported swimming, (2) any reported bicycling but no swimming, and (3) no exercise. We present data from early/mid-pregnancy and throughout pregnancy. The questionnaires are available at www.bsmb.dk.

All water activities were included in the swimming group, including antenatal water-exercises or baby-swimming with older children. Although our data did not include information on exercise intensity, we expect both the bicycling and the swimming groups to include all levels from mild to vigorous training.

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Other Variables

We obtained mother's age at conception and sex of the offspring from the National Discharge Registry. The first pregnancy interview provided data on prepregnancy body mass index, occupational status, gravidity, parity, previous spontaneous abortions, bleeding in early pregnancy, chronic diseases, uterine fibroids/malformations/cone biopsy, subfecundity, smoking, coffee and alcohol consumption, working hours, working position, physically strenuous work, and psychosocial jobstrain. The adjustment variables varied among the endpoint analyses and are displayed in the table footnotes. The decision on which factors to adjust for was made a priori, based upon existing knowledge regarding risk factors for a given end point.

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Reproductive Endpoints

We abstracted from the National Discharge Registry data on gestational age at birth (days), birth weight (g), length (cm), ponderal index (calculated as [weight in g × 100]/[length in cm3]),48 head circumference (cm), abdominal circumference (cm), placental weight (g), and congenital malformations. We defined preterm birth as a delivery after 153 gestational days (22 completed weeks) but before 259 days (37 completed weeks); postterm birth was a delivery at or after 294 gestational days (42 completed weeks). For SGA we used a 10% cutpoint of the sex- and gestation-specific birth weight within the Danish National Birth Cohort. Subgroups of congenital malformations were defined using ICD10 codes on malformations registered within the first year of life (the 10th revision of International Classification of Diseases). If a child had more than one malformation, the child was included in each subgroup.

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Statistical Analysis

We compared reproductive endpoints among swimmers, bicyclists, and nonexercisers. To evaluate the importance of time spent in swimming pools, we also calculated measures of association for 2 levels of time spent swimming and bicycling (<90 and 90+ minutes/week) relative to no exercise. We considered bicycling to be a similar exercise to swimming, in that both are nonweight-bearing sports that can be performed at a range of levels. We compared swimmers with nonexercisers to consider whether any possible effect of swimming on birth outcomes could be explained by physical activity in general, rather than to swimming specifically.

We estimated hazard ratios (HRs) and associated 95% confidence intervals (CIs) for preterm birth and SGA, using Cox regression analysis. If a second pregnancy interview was available, exercise data were updated by the time of the second interview. To adjust for the different entry times into follow-up, we stratified our models by gestational age at the time of the first and the second interview, respectively. For preterm birth, time at risk started from the day a woman completed the 22nd gestational week or on the day of her first pregnancy interview, whichever was later. Follow-up ended at birth or by the time a woman completed gestational week 37, whatever came first. For SGA analyses, the Cox model was constructed in the same way, except that time at risk was not restricted as described above. Hence, no time restrictions were applied to the analyses of SGA. We used linear regression models to calculate mean differences in birth weight, length, ponderal index, head and abdominal circumference, and placental weight. Logistic regression analysis was used to estimate odds ratios (ORs) for postterm birth, any congenital malformation, and subgroups of malformations.

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RESULTS

Table 1 shows the distribution of each outcome according to swimming, bicycling, and no exercise. In this population, restricted to women who were in one of these exercise categories, 75% did not engage in any exercise, 14% reported swimming, and 11% reported bicycling (and no swimming). There were fewer preterm births among swimming mothers compared with the other groups. Apart from this, there were only minor differences between swimmers and bicyclists. There were fewer postterm deliveries, more SGA-babies, and a slightly higher occurrence of malformations in the offspring among nonexercising mothers compared with swimmers and bicyclists.

TABLE 1

TABLE 1

Table 2 shows the association of swimming and bicycling in early/mid-pregnancy with preterm and postterm birth, SGA, congenital malformations, and fetal growth measures, compared with no exercise. We found a slightly decreased risk of preterm birth and a modestly decreased prevalence of overall congenital malformations among swimmers compared with nonexercisers. No major differences were observed between swimming and bicycling. Risk estimates for respiratory and cleft lip/palate malformations indicated a decreased risk among swimmers compared with nonexercisers, but estimates are based on small numbers with substantial uncertainty. Except for cleft lip/palate malformations, malformation estimates were similar among swimmers and bicyclists.

TABLE 2

TABLE 2

We also used a measure of swimming, bicycling, and no exercise, respectively, throughout pregnancy, ie reported at both the first and the second pregnancy interview (eTable, http://links.lww.com/EDE/A365). The results were very similar to those from early/mid-pregnancy.

In Table 3 the amount of swimming is evaluated. The table shows only minor differences between high and low levels of swimming, and, further, the relation between high levels of swimming and high levels of bicycling reflects the results presented in Table 2.

TABLE 3

TABLE 3

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DISCUSSION

This study does not indicate any adverse effects of swimming, or other water activities, in early/mid-pregnancy on preterm and postterm birth, SGA, birth weight and other fetal growth measures, or congenital malformations. Similarly reassuring results were found for those who swam throughout pregnancy and those who reported higher amounts of swimming, ie, at least 1.5 hours per week. These findings do not rule out the possibility that chemicals used in indoor pools are fetotoxic at higher exposure levels. Furthermore, although this is a large mother-child cohort, the study has limited power to detect an increased risk for rare outcomes such as specific congenital malformations. Our findings are in line with those of the only previously published study with a specific focus on swimming during pregnancy, by Nieuwenhuijsen et al,42 which indicated no association between swimming and birth weight. We did not observe marked differences in outcomes between swimmers and bicyclists, and the use of a secondary exercise group for comparison (bicyclists) strengthens our conclusions and further supports previous findings. Our data, however, suggest that physical exercise may be beneficial, or alternatively that the modestly elevated risk for preterm birth and perhaps congenital malformations seen among nonexercisers compared with swimmers could be related to medical reasons for not doing exercise (confounding by the indication for not being physically active). Reverse causation may also explain these differences if an affected fetus impairs maternal health during pregnancy, although this seems unlikely particularly in the early phase of pregnancy. Even though both swimming and bicycling have been found to increase uteroplacental vascular resistance, and thereby reduce blood flow to the uterus, the flow in the umbilical artery has been found to remain unchanged.49 This is consistent with the lack of substantial birth weight differences in offspring between the exercise groups and nonexercise group in our study.

Bicycling is a common means of commuting in Denmark, and many of the bicyclists in the study most likely used their bike for commuting as part of a daily routine. Since data on physical activity were both self-reported and did not include information on intensity, we expect the bicycling group to include all levels of intensity. Information on swimming faces a similar problem, because swimming and all other water activities were included. Subdividing the swimming and bicycling groups into 2 levels of weekly time spent on the activity did not change the results.

Even though we have considered bicycling a fair comparison with swimming in this paper, there may be arguments against this assumption. Bicycling is done in a sitting position, which might increase abdominal pressure on the pelvis; bicycling is perhaps more aerobic, because breathing is not as limited as it is during swimming; bicycling uses the lower body, which has larger musculature, while swimming may use more of the upper body; and swimming may allow for better heat dissipation from the mother's body. We have, however, not been able to identify another exercise group that would be a more appropriate comparison group.

Our study addresses exercise as commonly practiced in the population, and most of the participating women are not expected to be heavily exposed to swimming pool chemicals. There is almost no publicly available information on levels of disinfection by-products in swimming pools in Denmark. Studies of swimming pools in other countries, such as Spain, indicate that levels of trihalomethanes in swimming pools are not higher than those found in drinking water.50 Our findings suggest that the current practice of swimming in pools for pregnant women is safe for the endpoints we studied.

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ACKNOWLEDGMENTS

The Danish National Research Foundation has established the Danish Epidemiology Science Centre that initiated and created the Danish National Birth Cohort. The cohort is furthermore a result of a major grant from this foundation. Additional support for the Danish National Birth Cohort is obtained from the Pharmacy Foundation, the Egmont Foundation, the March of Dimes Birth Defects Foundation, and the Augustinus Foundation.

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