Abstract
Background
Little is known about the relationship between maternal occupational physical exertion and leisure time physical activity (LTPA) with preterm delivery (PTD) among Thai women, a population that differs in many respects from women in Western countries.
Objectives
To evaluate associations of maternal occupational physical exertion and LTPA with PTD in aggregate and in subgroups (i.e., spontaneous preterm labor, preterm premature rupture of membrane, medically indicated preterm delivery, moderate preterm delivery [gestational age 32–36 weeks], and very preterm delivery [<32 weeks]) among Thai women.
Methods
This case-control study included 467 PTD cases and 467 term controls. Maternal occupational exertion during pregnancy, as well as habitual engagement in LTPA before and during pregnancy was assessed using a structured questionnaire administered after delivery. Logistic regression procedures were used to examine relationships between both occupational and leisure time physical activity and PTD.
Results
After controlling for potential confounders, women who reported heavy physical occupational exertion during pregnancy, compared with other women, had a 2.42-fold increased risk of PTD overall [OR=2.42, 95% CI: 1.15, 5.09]. Very PTD [OR=4.57, 95% CI: 1.65, 12.64] and medically indicated PTD [OR=3.79, 95% CI: 1.54, 9.32] were particularly strongly associated with heavy occupational exertion. Maternal participation in LTPA before pregnancy was associated with a 24% reduction in PTD risk overall [OR=0.76, 95% CI: 0.57, 1.00], though no similar pattern in risk reduction was observed for LTPA performed during the first 6 months of pregnancy [OR=0.96, 95% CI: 0.68, 1.36].
Conclusion
Consistent with some previous reports, we noted that heavy occupational physical exertion is associated with at least a doubling in PTD risk. Patterns of associations of PTD risk with LTPA were less well delineated in this Thai population.
Keywords: Exercise, Occupational Activity, Physical Activity, Pregnancy, Preterm Delivery
Introduction
Preterm delivery (PTD), delivery before completion of 37 weeks of gestation, is a major determinant of maternal and neonatal morbidity and mortality worldwide [1–3]. Published studies, primarily from North America and Europe, have yielded inconsistent findings with some [4–8], but not all [9, 10] investigators reporting reductions in PTD risk associated with maternal habitual engagement in leisure time physical activity (LTPA). Similar inconsistencies have also been noted with regards to PTD risk in relation to maternal occupational physical activity or exertion. Some investigators have observed increased PTD risk with occupational physical exertion [11–16], whilst others have reported inverse or no associations of occupational activity with PTD risk [4, 8, 17]. We are aware of only one Thai study that assessed PTD risk in relation to maternal occupational exertion [18]. The authors reported that physical exertion (as characterized by hours worked per day, hours spent standing or squatting, walking pace at work and frequency of lifting objects weight 12 kg at work) was not associated with PTD risk [18]. With the exception of this single study, little is known about the relationship between maternal occupational exertion or LTPA and PTD among Thai women, a population that differs in many respects from women in Western countries. Therefore, we sought to better understand the risk of PTD in relation to maternal early pregnancy occupational physical exertion and LTPA (performed both before and during pregnancy) among Thai women.
Methods
Study population and data collection
A case-control study using one control for each case of PTD was conducted among women who delivered live births at King Chulalongkorn Memorial Hospital, Rajavithi Hospital, and Police General Hospital, Bangkok, Thailand between July 2006 and November 2007. Cases were women with singleton pregnancies who delivered before 37 completed weeks of gestation (22–36 weeks of gestation). PTD cases were identified by daily monitoring of all new deliveries at postpartum wards of participating hospitals. Of the 478 eligible cases approached, 467 (97.7%) agreed to participate in the study. Controls were women who delivered a singleton infant at term (≥37 weeks of gestation) and were selected from the same hospital of delivery. An eligible control, delivering immediately after a case patient, was approached and recruited for the study. Of the 482 eligible controls approached, 467 (96.9%) agreed to participate in the study.
All participants provided informed consent and the research protocol was reviewed and approved by ethical committees of the Faculty of Medicine, Chulalongkorn University, Rajavithi Hospital, Police General Hospital and the Institutional Review Boards, Division of Human Subjects Research, University of Washington.
After obtaining informed consent, enrolled participants were asked to participate in a 45-minute in-person interview in which trained research personnel used a structured questionnaire to elicit information regarding maternal socio-demographic, lifestyle, medical and reproductive characteristics. We asked women to specify whether they were employed outside of the household during pregnancy. We further asked women to specify how much physical exertion was involved in their jobs during pregnancy. Levels of exertion were as follows: (1) very light exertion such as sitting during performance of routine office work; (2) relatively light exertion such as standing but seldom lifting or carrying objects; (3) medium exertion such as lifting and carrying objects weighing more than 5 kg, climbing stairs while engaged in work involving house cleaning or child care; (4) considerable exertion such as any physical labor that causes shortness of breath; and (5) heavy exertion such as physically stressful work including lifting and carrying objects or loads >25kg as in construction-related work. We queried all women about their usual participation in leisure time physical activity (LTPA) before and during pregnancy; and we asked active women to report the number of days per week and hours per day they participated in LTPA.
Participants’ labor and delivery medical records and prenatal medical records were also reviewed by trained obstetric research nurses who used a standardized abstraction form. Information abstracted from medical records included participants’ pre-pregnancy weight, height, blood pressure, pregnancy complications and condition of the newborn.
Analytical variable specification
Preterm delivery
The diagnosis of PTD was made using American College of Obstetricians and Gynecologists (ACOG) guidelines [19]. Gestational age was based on the last menstrual period (LMP) or ultrasound examination. If both LMP and ultrasound dating (before 20 weeks gestation) were available and the two agreed within 14 days, we used the former to assign gestational age. If the two dates differed by more than 14 days, we used the ultrasound date. Using detailed information collected from medical records, we categorized PTD cases according to the three pathophysiological groups previously described (i.e., spontaneous preterm labor and delivery, preterm premature rupture of membranes, and medically indicated preterm delivery) [3, 20]. Spontaneous preterm labor and delivery cases were comprised of women whose medical records indicated a physician diagnosis of spontaneous labor onset (with intact fetal membranes) and delivery prior to the completion of 37 weeks gestation. Preterm premature rupture of membranes cases were comprised of women whose medical records indicated a physician diagnosis of rupture of fetal membranes (prior to the onset of labor) and delivery prior to the completion of 37 weeks gestation.
Women who delivered prior to 37 completed weeks of gestation as a result of medical intervention comprised the medically indicated preterm delivery group. We also categorized PTD cases according to gestational age at delivery (i.e., very preterm delivery, defined as delivery prior to the completion of 32 weeks gestation; and moderate preterm delivery, defined as delivery between 32 and 36 weeks).
Physical activity variables
Women were categorized into three groups with respect to physical exertion related to occupational activity during pregnancy: Light exertion, medium and heavy exertion, respectively. We also considered PTD risk in relation to whether women reported engaging in leisure time physical activity before and during pregnancy, respectively. Lastly, we sought to determine the joint effects of leisure time activity before and during pregnancy in relation to PTD risk. For this analysis, we categorized women into four groups (inactive both time periods, physically active before pregnancy only, physically active during pregnancy only, and physically active before and during pregnancy).
Other covariates
Covariates considered in this analysis included maternal socio-demographic characteristics and behavioral including maternal age, marital status, educational attainment, employment status, cigarette smoking and alcohol consumption during pregnancy. Also considered were maternal reproductive and medical histories including parity, prior history of PTD, history of abortion, maternal height, weight and infant gender. Parity was reported as the number of previous pregnancies lasting more than 22 weeks gestation. Pre-pregnancy body mass index was calculated as weight (in kilograms) divided by the square of height (in meters).
Statistical analysis
We examined frequency distributions of maternal characteristics according to preterm and term delivery status. To assess the relation between physical activity measures and preterm delivery risk, we fit multivariable logistic regression models and estimated adjusted odds ratios (OR) and 95% confidence intervals (CI) [21, 22]. To assess confounding, we entered covariates into each model one at a time and compared the adjusted and unadjusted odds ratios. Final models included covariates that altered unadjusted odds ratios by at least 10% [22] and those of a priori interest (i.e., maternal age and parity). We considered the covariates listed in Table 1 as potential confounders. Because pre-pregnancy adiposity may be either an intermediate outcome in the causal pathway or a confounder in the relation between physical activity and preterm delivery risk, we evaluated models with and without adjustment for pre-pregnancy body mass index (BMI). Inclusion of pre-pregnancy BMI into final model did not materially alter associations, so to be consistent with several other published studies[5, 7, 9, 10, 17]we report models that include this covariate. These analytical procedures were also used in stratified analyses designed to assess risk of subtypes of preterm delivery (i.e., spontaneous preterm labor and delivery, preterm premature rupture of membranes, medically indicated preterm delivery, moderate preterm delivery, and very preterm delivery). All analyses were completed using SPSS, version 16.0 statistical software (SPSS Inc., Chicago, IL).
Table 1.
Characteristics of Term Controls and Preterm Cases, Bangkok, Thailand, 2006–2007.
| Maternal Characteristics | Controls (n = 467) | Preterm Cases (n = 467) | ||
|---|---|---|---|---|
| na | % | na | % | |
| Maternal age (years) | ||||
| <20 | 63 | 13.5 | 81 | 17.3 |
| 20–24 | 130 | 27.8 | 129 | 27.6 |
| 25–29 | 145 | 31.0 | 110 | 23.6 |
| 30–34 | 83 | 17.8 | 86 | 18.4 |
| ≥35 | 46 | 9.9 | 61 | 13.1 |
| Maternal education (years) | ||||
| ≤6 | 153 | 32.8 | 155 | 33.2 |
| 7–12 | 282 | 60.4 | 278 | 59.5 |
| >12 | 32 | 6.9 | 34 | 7.3 |
| Marital status | ||||
| Married | 231 | 49.5 | 229 | 49.0 |
| Living with boyfriend/partner | 219 | 46.9 | 208 | 44.5 |
| Separated | 17 | 3.6 | 30 | 6.4 |
| Parity | ||||
| Nulliparous | 249 | 53.3 | 275 | 58.9 |
| Multiparous | 218 | 46.7 | 192 | 41.1 |
| Smoke during pregnancy | ||||
| No | 459 | 98.3 | 455 | 97.4 |
| Yes | 8 | 1.7 | 12 | 2.6 |
| Alcohol use in pregnancy | ||||
| No | 445 | 95.3 | 451 | 96.6 |
| Yes | 22 | 4.7 | 16 | 3.4 |
| Pre-pregnancy BMI (kg/m2) | ||||
| Underweight (<18.5) | 80 | 17.6 | 119 | 26.9 |
| Normal (18.5–24.9) | 315 | 69.4 | 268 | 60.5 |
| Overweight (25.0–29.9) | 45 | 9.9 | 38 | 8.6 |
| Obesity (≥30.0) | 14 | 3.1 | 18 | 4.1 |
| Onset of prenatal care | ||||
| Care initiated in 1st trimester | 199 | 42.6 | 159 | 34.0 |
| Care initiated after 1st trimester | 256 | 54.8 | 267 | 57.2 |
| No prenatal care | 12 | 2.6 | 41 | 8.8 |
| Prior history of preterm delivery | ||||
| Nulliparous | 249 | 53.3 | 275 | 58.9 |
| Parous-no prior preterm delivery | 204 | 43.7 | 149 | 31.9 |
| Parous-prior preterm delivery | 14 | 3.0 | 43 | 9.2 |
Number may not be added up to the total number due to missing data
Results
Socio-demographic and reproductive characteristics of cases and controls are presented in Table 1. Compared with term controls, PTD cases were more likely to be underweight and have received no prenatal care during the index pregnancy. Cases were also more likely to have had a previous preterm delivery than controls.
Heavy occupational physical exertion during pregnancy was associated with a 2.30-fold increased risk of PTD [95% CI: 1.10, 4.83] (Table 2). Adjustment for possible confounding by maternal age, pre-pregnancy body mass index, and parity did not substantially alter the magnitude of the observed association [adjusted OR=2.42, 95% CI: 1.15, 5.09]. After adjusting for confounders, LTPA performed before pregnancy was associated with a 24% reduced risk of PTD [adjusted OR=0.76, 95% CI: 0.57, 1.00]. LTPA performed during pregnancy, however, appeared not to be associated with PTD risk [adjusted OR=0.96, 95% CI: 0.68, 1.36]. When we evaluated maternal habitual LTPA before and during pregnancy, we noted that persistent exercisers (those who exercised before and during pregnancy), as compared with consistently sedentary women (those who were inactive before and during pregnancy: the referent group) had a 15% reduction in risk of PTD [adjusted OR=0.85, 95% CI: 0.59, 1.24]. Women who only engaged in LTPA before pregnancy, as compared with consistently sedentary women, had a 31% reduction in risk of PTD [adjusted OR=0.69, 95% CI: 0.48, 0.89].
Table 2.
Unadjusted and Adjusted Odds Ratios (OR) and 95% Confidence Intervals (95% CI) for Preterm Delivery According to Occupational and Leisure Time Physical Activity Performed Before and/or During Pregnancy, Bangkok, Thailand, 2006–2007.
| Measurement | Term Controls (n = 467) | Preterm Cases (n = 467) | Unadjusted OR [95% CI] | Adjusted ORb [95% CI] | ||||
|---|---|---|---|---|---|---|---|---|
|
| ||||||||
| na | % | na | % | |||||
| Occupational physical exertion during pregnancyc | ||||||||
| Light exertion | 145 | 50.2 | 149 | 50.3 | 1.00 | Reference | 1.00 | Reference |
| Medium exertion | 133 | 46.0 | 121 | 40.9 | 0.89 | [0.63, 1.24] | 0.90 | [0.64, 1.27] |
| Heavy exertion | 11 | 3.8 | 26 | 8.8 | 2.30 | [1.10, 4.83] | 2.42 | [1.15, 5.09] |
| Leisure-time physical activity before pregnancy | ||||||||
| No | 277 | 60.6 | 305 | 66.6 | 1.00 | Reference | 1.00 | Reference |
| Yes | 180 | 39.4 | 153 | 33.4 | 0.77 | [0.59, 1.01] | 0.76 | [0.57, 1.00] |
| Leisure-time physical activity during pregnancy | ||||||||
| No | 371 | 82.3 | 372 | 81.9 | 1.00 | Reference | 1.00 | Reference |
| Yes | 80 | 17.7 | 82 | 18.1 | 1.02 | [0.73, 1.44] | 0.96 | [0.68, 1.36] |
| Leisure-time physical activity | ||||||||
| Inactive both before and during pregnancy | 272 | 60.4 | 292 | 65.5 | 1.00 | Reference | 1.00 | Reference |
| Active only during pregnancy | 5 | 1.1 | 9 | 2.0 | 1.68 | [0.56, 5.07] | 1.29 | [0.40, 4.13] |
| Active only before pregnancy | 98 | 21.8 | 73 | 16.4 | 0.69 | [0.49, 0.98] | 0.69 | [0.48, 0.98] |
| Active both before and during pregnancy | 75 | 16.7 | 72 | 16.1 | 0.89 | [0.62, 1.29] | 0.85 | [0.59, 1.24] |
Number may not be added up to the total number due to missing data
Adjusted for maternal age, pre-pregnancy body mass index, and parity
Analysis were restricted to women who were employed during pregnancy
We next evaluated risk of SPTD, PPROM, and MIPTD, respectively in relation to maternal occupational exertion and LTPA (Table 3). Heavy occupational exertion was associated with a 2.07-fold increased risk of SPTD [adjusted OR=2.07, 95% CI: 0.81, 5.28] and a 3.79-fold increased risk of MIPTD [adjusted OR=3.79, 95% CI: 1.54, 9.32]. No such association was observed for PPROM [adjusted OR=0.86, 95% CI: 0.22, 3.27]. LTPA performed before pregnancy was associated with 30%, 21%, and a 14% reduction in risk of SPTD, PPROM, MIPTD, respectively, though these associations did not reach statistical significance. LTPA performed before pregnancy only or performed consistently before and during pregnancy was associated with reduction in risks for the three clinical subgroups of PTD. None of these associations, however, reached statistical significance after adjustments for confounding were made.
Table 3.
Adjusted Odds Ratios (OR) and 95% Confidence Intervals (95% CI) for Preterm Delivery Subtypes According to Occupational and Leisure Time Physical Activity Performed Before and/or During Pregnancy, Bangkok, Thailand, 2006–2007.
| Measurement | Term Controls (n = 467)
|
Spontaneous Preterm Delivery (n = 230)
|
Preterm Premature Rupture of Membrane (n = 120)
|
Medically Indicated Preterm Delivery (n = 117)
|
||||||
|---|---|---|---|---|---|---|---|---|---|---|
| na | na | ORb | [95% CI] | na | ORb | [95% CI] | na | ORb | [95% CI] | |
| Occupational physical exertion during pregnancyc | ||||||||||
| Light exertion | 145 | 62 | 1.00 | Reference | 39 | 1.00 | Reference | 48 | 1.00 | Reference |
| Medium exertion | 133 | 55 | 0.94 | [0.60, 1.48] | 40 | 1.12 | [0.66, 1.89] | 26 | 0.60 | [0.34, 1.06] |
| Heavy exertion | 11 | 10 | 2.07 | [0.81, 5.28] | 3 | 0.86 | [0.22, 3.27] | 13 | 3.79 | [1.54, 9.32] |
| Leisure-time physical activity before pregnancy | ||||||||||
| No | 277 | 152 | 1.00 | Reference | 76 | 1.00 | Reference | 77 | 1.00 | Reference |
| Yes | 180 | 73 | 0.70 | [0.49, 1.00] | 41 | 0.79 | [0.51, 1.23] | 39 | 0.86 | [0.55, 1.36] |
| Leisure-time physical activity during pregnancy | ||||||||||
| No | 371 | 183 | 1.00 | Reference | 95 | 1.00 | Reference | 94 | 1.00 | Reference |
| Yes | 80 | 41 | 0.94 | [0.61, 1.46] | 21 | 1.04 | [0.60, 1.79] | 20 | 0.86 | [0.48, 1.54] |
| Leisure-time physical activity | ||||||||||
| Inactive both before and during pregnancy | 272 | 143 | 1.00 | Reference | 74 | 1.00 | Reference | 75 | 1.00 | Reference |
| Active only during pregnancy | 5 | 6 | 1.55 | [0.44, 5.49] | 2 | 1.43 | [0.26, 7.72] | 1 | - | |
| Active only before pregnancy | 98 | 35 | 0.65 | [0.41, 1.03] | 20 | 0.70 | [0.40, 1.24] | 18 | 0.81 | [0.45, 1.47] |
| Active both before and during pregnancy | 75 | 35 | 0.82 | [0.51, 1.31] | 18 | 0.88 | [0.49, 1.59] | 19 | 0.88 | [0.48, 1.61] |
Number may not be added up to the total number due to missing data
Adjusted for maternal age, pre-pregnancy body mass index, and parity
Analysis were restricted to women who were employed during pregnancy
Finally, we assessed risk of moderate (32–36 weeks) and very PTD (<32 weeks) in relation to maternal occupational exertion and LTPA. As shown in Table 4, heavy occupational exertion was associated a 4.57-fold increased risk of very PTD [adjusted OR=4.57, 95% CI: 1.65, 12.64]. Heavy occupational exertion was only moderately and statistically insignificantly associated with moderate PTD [adjusted OR=1.94, 95% CI: 0.88, 4.29]. LTPA performed before pregnancy only [adjusted OR=0.67, 95% CI: 0.46, 0.97] or performed consistently before and during pregnancy [adjusted OR=0.78, 95% CI: 0.53, 1.16] were associated with reductions in risks of moderate PTD, though only the former reached statistical significance. Inferences from analyses restricted to very PTD are hindered by the small sample size.
Table 4.
Adjusted Odds Ratios (OR) and 95% Confidence Intervals (95% CI) for Moderate and Very Preterm Delivery (PTD) According to Occupational and Leisure Time Physical Activity Performed Before and/or During Pregnancy, Bangkok, Thailand, 2006–2007.
| Measurement | Term Controls (n = 467)
|
Moderate PTD (32–36 wks) (n = 389)
|
Very PTD (<32 wks) (n = 78)
|
||||
|---|---|---|---|---|---|---|---|
| na | na | ORb | [95% CI] | na | ORb | [95% CI] | |
| Occupational physical exertion during pregnancyc | |||||||
| Light exertion | 145 | 124 | 1.00 | Reference | 25 | 1.00 | Reference |
| Medium exertion | 133 | 101 | 0.89 | [0.62, 1.28] | 20 | 0.98 | [0.50, 1.90] |
| Heavy exertion | 11 | 18 | 1.94 | [0.88, 4.29] | 8 | 4.57 | [1.65, 12.64] |
| Leisure-time physical activity before pregnancy | |||||||
| No | 277 | 261 | 1.00 | Reference | 44 | 1.00 | Reference |
| Yes | 180 | 121 | 0.70 | [0.52, 0.94] | 32 | 1.19 | [0.68, 1.91] |
| Leisure-time physical activity during pregnancy | |||||||
| No | 371 | 315 | 1.00 | Reference | 57 | 1.00 | Reference |
| Yes | 80 | 64 | 0.91 | [0.63, 1.32] | 18 | 1.30 | [0.68, 2.39] |
| Leisure-time physical activity | |||||||
| Inactive both before and during pregnancy | 272 | 249 | 1.00 | Reference | 43 | 1.00 | Reference |
| Active only during pregnancy | 5 | 8 | 1.53 | [0.48, 4.92] | 1 | - | |
| Active only before pregnancy | 98 | 61 | 0.67 | [0.46, 0.97] | 12 | 0.88 | [0.42, 1.67] |
| Active both before and during pregnancy | 75 | 55 | 0.78 | [0.53, 1.16] | 17 | 1.36 | [0.69, 2.57] |
Number may not be added up to the total number due to missing data
Adjusted for maternal age, pre-pregnancy body mass index, and parity
Analysis were restricted to women who were employed during pregnancy
Discussion
Heavy occupational physical exertion was associated with an approximate doubling in risk of PTD. This increased risk was particularly pronounced for very PTD delivery [adjusted OR=4.57, 95% CI: 1.65, 12.64], and MIPTD [adjusted OR=3.79, 95% CI: 1.54, 9.32]. In addition, women who engaged in LTPA both before and during pregnancy had a 15% reduced risk of PTD compared with women who were inactive during both time periods [adjusted OR=0.85, 95% CI: 0.59, 1.24]. However, this association did not reach statistical significance.
Numerous studies have examined PTD risk in relation to maternal occupational physical activity and/or exertion. A recent meta-analysis of 35 published studies indicated that heavy maternal occupational exertion was associated with at least a 40% increase in PTD risk [11]. The authors’ overall conclusions are generally consistent with the 2.42-fold increased PTD risk associated with heavy occupational exertion observed in our study. However, the literature on this area of research has been inconsistent. Some [12–16], though not all [4, 8, 17, 18], investigators have documented positive associations between PTD risk and occupational physical activity. Luke et al. [12], in their case-control study of nurses, reported positive associations of PTD risk with various aspects of occupational exertion. For instance, the authors reported a more than doubling in risk of PTD [adjusted OR=1.7] associated with standing for 4–6 hours per shift (a shift was defined between 4 and 12 hours). Additionally, the authors reported that nurses who had a high occupational fatigue score had a 40% increased PTD risk [adjusted OR=1.4, 95% CI: 1.1, 1.9], as compared with others [12]. Collectively, observations reported by Luke et al. and others [12–16] are consistent with our observed 2.42-fold increased risk of PTD among women reporting heavy occupational exertion during pregnancy. Our results, however, are inconsistent with reports by other investigators [4, 8, 17, 18]. Misra et al. [8] specified lifting heavy objects at work and standing/moving on the job (hrs/week) as characteristics of maternal occupational physical activity levels. In their study of American women, neither lifting heavy objects nor standing/moving on the job for ≥21 hours per week were risk factors of PTD [8]. Tuntiseranee et al. [18], in their study of 1,797 women from southern Thailand characterized maternal occupational exertion using a series of covariates and found no positive associations PTD risk. The authors estimated the amount of squatting (hrs/day), standing (hrs/day), fast walking, lifting more than 12kg (times/day), lifting level, carrying more than 12kg (times/day), and physical job demand associated with maternal occupations. However none of these covariates were found to be risk factors of PTD. Differences in study populations, methods used to ascertain and define occupational activity, selection bias, errors in recall and residual confounding may contribute to variations in findings across studies. Variations in the type, intensity, and timing of occupational physical activities in relation to the pregnancy are also likely explanations for differences in results across studies.
Maternal LTPA performed before pregnancy was associated with a 24% reduction in risk of PTD overall; reductions of similar magnitude were also observed for SPTD, PPROM, MIPTD, and moderate PTD. Overall these results are consistent with findings reported by Berkowitz et al. [4] Berkowitz et al. [4] reported a reduction in risk of PTD [OR=0.67, 95% CI: 0.46, 0.97] with LTPA performed one year before pregnancy. However, our results and those of Berkowitz et al. [4] are inconsistent with findings reported by Evenson et al. [5], who found no association between PTD risk and maternal LTPA performed three months prior to pregnancy [OR=1.06, 95% CI: 0.71, 1.57]. Differences in population characteristics, methods of ascertaining maternal LTPA, and incomplete control of confounding may contribute to inconsistencies across studies.
In the present study, we found that LTPA performed both before and during pregnancy was associated with a decreased risk of PTD [adjusted OR=0.85, 95% CI: 0.59, 1.24]. This finding is largely consistent with reports from other studies [4–6, 10]. For example, Hatch et al. (1998) reported that women who engaged in LTPA both before and during pregnancy had an 89% reduced risk of any PTD [RR=0.11, 95% CI: 0.02, 0.81]. Lieferman and Evenson [10], in their study of 9,089 women, reported that those who did not participate in LTPA before or during pregnancy had an 11% increased PTD risk compared to women who exercised both before and during pregnancy [OR=1.11, 95% CI: 0.93, 1.31].
Our study has several strengths, including the relatively large sample of PTD cases, and the high participation rate of cases and controls (97.7% and 96.9% respectively). These high participation rates serve to attenuate concerns about selection bias. Several limitations, however, should be considered when interpreting study findings. First, our case-control study relies on self-reported physical activity and self-assessment of occupational physical exertion. As a result, we cannot exclude the possibility that our findings may be influenced by recall bias. However, we used well-trained interviewers, blinded to participants’ case-control status, and employed a structured questionnaire to collect information concerning maternal physical activity. Together, these aspects of our study served to attenuate some concerns about systematic errors in data collection. Second, although we evaluated and adjusted for potential confounders, we cannot exclude the possibility that some residual confounding by covariates not measured in our study (e.g. a healthy diet, use of multivitamins) may have influenced reported odds ratios. Lastly, limited statistical power, particularly for our subgroup analysis, resulted in relatively imprecise estimates of relative risk as reflected by our wide 95% confidence intervals.
A number of biologically plausible hypotheses in support of PTD risk reduction in relation to maternal LTPA have been discussed by prior investigators [4–8]. For example, investigators have noted that habitual exercise, performed by men and non-pregnant women, favorably alters plasma lipid and lipoproteins concentrations, diastolic and systolic blood pressures, endothelial function, glycemic control, immunological parameters, and adipocytokines [23–25]. Notably, alterations in the aforementioned metabolic pathways are of etiologic importance in PTD [26, 27], particularly MIPTD which may occur as a clinical consequence of maternal hypertensive disorders, including preeclampsia [28, 29]. A small number of investigators have documented the impact of physical activity during pregnancy on maternal inflammatory [30], endocrine [25], blood pressure [31] and lipid [24] profiles. Collectively, available data provide several promising mechanistic hypotheses (for the observed statistical associations) that require rigorous testing in well designed clinical investigations.
Occupational exertion, unlike LTPA, was associated with an increased risk of PTD. LTPA may be fundamentally different from both physical activity of daily living (i.e. stair climbing, walking for a purpose) and occupational physical activity (i.e. heavy lifting, long periods of standing) [32]. For example, Misra et al. [8] have speculated that LTPA is performed under less stressful circumstances and may be less strenuous than activities of daily living, such as purposeful walking, and physical exertion at work. Moreover, other investigators [28, 32, 33] have noted that standing for long periods of time, for example, may disrupt utero-placental blood flow, alter hormone balance, and increase intra-abdominal pressure. These physiological changes may contribute to adverse maternal and fetal outcomes such as poor placental perfusion, premature labor and restricted fetal growth.
In summary, we noted that heavy occupational physical activity was associated with a significantly increased risk of PTD, especially MIPTD and very PTD among Thai women. Maternal participation in LTPA before pregnancy was associated with a 24% reduction in PTD risk overall though no similar pattern in risk reduction was observed for LTPA performed during the first 6 months of pregnancy. More studies are needed to further evaluate PTD risk in relation to maternal physical activity. Inferences from such studies will be enhanced by using a prospective cohort design that diminishes concerns about recall bias. Future studies should also incorporate objective measures of maternal physical activity and measures of maternal fitness before and during early pregnancy; and should be sufficiently large to allow for more precise estimates of clinical subtypes of PTD in relation to physical activity.
Acknowledgments
This research was supported by the Rachadapiseksompoj Faculty of Medicine Research Fund (RA 20/49), Chulalongkorn University and the Multidisciplinary International Research Training (MIRT) Program of the University of Washington, School of Public Health and Community Medicine. The MIRT Program is supported by an award from the National Institutes of Health, National Center on Minority Health and Health Disparities (T37-MD001449). The authors wish to thank the staff nurses at King Chulalongkorn Memorial Hospital, Rajavithi Hospital and Police General Hospital in Bangkok, Thailand for their assistance with data collection.
Abbreviations and units
- OR
Odds ratio
- 95% CI
95% confidence interval
- PTD
Preterm delivery
- SPTD
Spontaneous preterm labor and delivery
- PPROM
Preterm premature rupture of membrane
- MIPTD
Medically indicated preterm delivery
- LTPA
Leisure-time physical activity
- LMP
Last menstrual period
- BMI
Body mass index
- kg
Kilograms
- m
meters
References
- 1.Institute of Medicine. Preterm Birth: Causes, Consequences, and Prevention. Institute of Medicine of The National Academies; Washington, DC: 2006. [Google Scholar]
- 2.Pennell CE, Jacobsson B, Williams SM, Buus RM, Muglia LJ, Dolan SM, et al. Genetic epidemiologic studies of preterm birth: guidelines for research. Am J Obstet Gynecol. 2007;196:107–118. doi: 10.1016/j.ajog.2006.03.109. [DOI] [PubMed] [Google Scholar]
- 3.Savitz DA, Blackmore CA, Thorp JM. Epidemiologic characteristics of preterm delivery: etiologic heterogeneity. Am J Obstet Gynecol. 1991;164:467–471. doi: 10.1016/s0002-9378(11)80001-3. [DOI] [PubMed] [Google Scholar]
- 4.Berkowitz GS, Kelsey JL, Holford TR, Berkowitz RL. Physical activity and the risk of spontaneous preterm delivery. J Reprod Med. 1983;28:581–588. [PubMed] [Google Scholar]
- 5.Evenson KR, Siega-Riz AM, Savitz DA, Leiferman JA, Thorp JM., Jr Vigorous leisure activity and pregnancy outcome. Epidemiology. 2002;13:653–659. doi: 10.1097/00001648-200211000-00009. [DOI] [PubMed] [Google Scholar]
- 6.Hatch M, Levin B, Shu XO, Susser M. Maternal leisure-time exercise and timely delivery. Am J Public Health. 1998;88:1528–1533. doi: 10.2105/ajph.88.10.1528. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 7.Hegaard HK, Hedegaard M, Damm P, Ottesen B, Petersson K, Henriksen TB. Leisure time physical activity is associated with a reduced risk of preterm delivery. Am J Obstet Gynecol. 2008;198:180, e181–185. doi: 10.1016/j.ajog.2007.08.038. [DOI] [PubMed] [Google Scholar]
- 8.Misra DP, Strobino DM, Stashinko EE, Nagey DA, Nanda J. Effects of physical activity on preterm birth. Am J Epidemiol. 1998;147:628–635. doi: 10.1093/oxfordjournals.aje.a009503. [DOI] [PubMed] [Google Scholar]
- 9.Barakat R, Stirling JR, Lucia A. Does exercise training during pregnancy affect gestational age? A randomised, controlled trial. Br J Sports Med. 2008 doi: 10.1136/bjsm.2008.047837. [DOI] [PubMed] [Google Scholar]
- 10.Leiferman JA, Evenson KR. The effect of regular leisure physical activity on birth outcomes. Matern Child Health J. 2003;7:59–64. doi: 10.1023/a:1022545718786. [DOI] [PubMed] [Google Scholar]
- 11.Bonzini M, Coggon D, Palmer KT. Risk of prematurity, low birthweight and pre-eclampsia in relation to working hours and physical activities: a systematic review. Occup Environ Med. 2007;64:228–243. doi: 10.1136/oem.2006.026872. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 12.Luke B, Mamelle N, Keith L, Munoz F, Minogue J, Papiernik E, et al. The association between occupational factors and preterm birth: a United States nurses’ study. Research Committee of the Association of Women’s Health, Obstetric, and Neonatal Nurses. Am J Obstet Gynecol. 1995;173:849–862. doi: 10.1016/0002-9378(95)90354-2. [DOI] [PubMed] [Google Scholar]
- 13.Mamelle N, Laumon B, Lazar P. Prematurity and occupational activity during pregnancy. Am J Epidemiol. 1984;119:309–322. doi: 10.1093/oxfordjournals.aje.a113750. [DOI] [PubMed] [Google Scholar]
- 14.Nguyen N, Savitz DA, Thorp JM. Risk factors for preterm birth in Vietnam. Int J Gynaecol Obstet. 2004;86:70–78. doi: 10.1016/j.ijgo.2004.04.003. [DOI] [PubMed] [Google Scholar]
- 15.Saurel-Cubizolles MJ, Kaminski M. Pregnant women’s working conditions and their changes during pregnancy: a national study in France. Br J Ind Med. 1987;44:236–243. doi: 10.1136/oem.44.4.236. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 16.Teitelman AM, Welch LS, Hellenbrand KG, Bracken MB. Effect of maternal work activity on preterm birth and low birth weight. Am J Epidemiol. 1990;131:104–113. doi: 10.1093/oxfordjournals.aje.a115463. [DOI] [PubMed] [Google Scholar]
- 17.Pompeii LA, Savitz DA, Evenson KR, Rogers B, McMahon M. Physical exertion at work and the risk of preterm delivery and small-for-gestational-age birth. Obstet Gynecol. 2005;106:1279–1288. doi: 10.1097/01.AOG.0000189080.76998.f8. [DOI] [PubMed] [Google Scholar]
- 18.Tuntiseranee P, Geater A, Chongsuvivatwong V, Kor-anantakul O. The effect of heavy maternal workload on fetal growth retardation and preterm delivery. A study among southern Thai women. J Occup Environ Med. 1998;40:1013–1021. doi: 10.1097/00043764-199811000-00013. [DOI] [PubMed] [Google Scholar]
- 19.ACOG technical bulletin. Preterm labor. Number 206--June 1995 (Replaces No 133, October 1989) Int J Gynaecol Obstet. 1995;50:303–313. [PubMed] [Google Scholar]
- 20.Williams MA, Mittendorf R, Stubblefield PG, Lieberman E, Schoenbaum SC, Monson RR. Cigarettes, coffee, and preterm premature rupture of the membranes. Am J Epidemiol. 1992;135:895–903. doi: 10.1093/oxfordjournals.aje.a116385. [DOI] [PubMed] [Google Scholar]
- 21.Breslow NE, Day NE, Gart JJ. Statistical methods in cancer research. Lyon: International Agency for Research on Cancer; 1980. International Agency for Research on Cancer. [Google Scholar]
- 22.Rothman KJ, Greenland S. Modern epidemiology. 2. Philadelphia, PA: Lippincott-Raven; 1998. [Google Scholar]
- 23.Department of Health and Human Services. Physical Activity and Health: A Report of the Surgeon General. National Cancer Center for Chronic Disease Prevention and Health Promotion; Atlanta, Georgia, United States: 1996. [Google Scholar]
- 24.Butler CL, Williams MA, Sorensen TK, Frederick IO, Leisenring WM. Relation between maternal recreational physical activity and plasma lipids in early pregnancy. Am J Epidemiol. 2004;160:350–359. doi: 10.1093/aje/kwh223. [DOI] [PubMed] [Google Scholar]
- 25.Ning Y, Williams MA, Butler CL, Muy-Rivera M, Frederick IO, Sorensen TK. Maternal recreational physical activity is associated with plasma leptin concentrations in early pregnancy. Hum Reprod. 2005;20:382–389. doi: 10.1093/humrep/deh615. [DOI] [PubMed] [Google Scholar]
- 26.Goldenberg RL, Culhane JF, Iams JD, Romero R. Epidemiology and causes of preterm birth. Lancet. 2008;371:75–84. doi: 10.1016/S0140-6736(08)60074-4. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 27.Romero R, Espinoza J, Kusanovic JP, Gotsch F, Hassan S, Erez O, et al. The preterm parturition syndrome. BJOG. 2006;113 (Suppl 3):17–42. doi: 10.1111/j.1471-0528.2006.01120.x. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 28.Dempsey JC, Butler CL, Williams MA. No need for a pregnant pause: physical activity may reduce the occurrence of gestational diabetes mellitus and preeclampsia. Exerc Sport Sci Rev. 2005;33:141–149. doi: 10.1097/00003677-200507000-00007. [DOI] [PubMed] [Google Scholar]
- 29.Sorensen TK, Williams MA, Lee IM, Dashow EE, Thompson ML, Luthy DA. Recreational physical activity during pregnancy and risk of preeclampsia. Hypertension. 2003;41:1273–1280. doi: 10.1161/01.HYP.0000072270.82815.91. [DOI] [PubMed] [Google Scholar]
- 30.Clapp JF, 3rd, Kiess W. Effects of pregnancy and exercise on concentrations of the metabolic markers tumor necrosis factor alpha and leptin. Am J Obstet Gynecol. 2000;182:300–306. doi: 10.1016/s0002-9378(00)70215-8. [DOI] [PubMed] [Google Scholar]
- 31.Yeo S, Steele NM, Chang MC, Leclaire SM, Ronis DL, Hayashi R. Effect of exercise on blood pressure in pregnant women with a high risk of gestational hypertensive disorders. J Reprod Med. 2000;45:293–298. [PubMed] [Google Scholar]
- 32.Clapp JF., 3rd Pregnancy outcome: physical activities inside versus outside the workplace. Semin Perinatol. 1996;20:70–76. doi: 10.1016/s0146-0005(96)80059-0. [DOI] [PubMed] [Google Scholar]
- 33.Naeye RL. Factors that predispose to premature rupture of the fetal membranes. Obstet Gynecol. 1982;60:93–98. [PubMed] [Google Scholar]