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American Journal of Epidemiology Advance Access published online on July 25, 2007
American Journal of Epidemiology, doi:10.1093/aje/kwm171
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American Journal of Epidemiology © The Author 2007. Published by the Johns Hopkins Bloomberg School of Public Health. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org.

Original Contribution

Work Activity in Pregnancy, Preventive Measures, and the Risk of Preterm Delivery

Agathe Croteau1,2, Sylvie Marcoux3 and Chantal Brisson3

1 National Institute of Public Health of Québec, Québec, QC, Canada
2 CHUQ Research Center, Laval University, Québec, QC, Canada
3 Department of Social and Preventive Medicine, Faculty of Medicine, Laval University, Québec, QC, Canada

Correspondence to Dr. Agathe Croteau, National Institute of Public Health of Québec, 945 Wolfe Avenue, Second Floor, Québec, QC, Canada G1V 5B3 (e-mail: agathe.croteau{at}inspq.qc.ca).

Received for publication November 3, 2006. Accepted for publication May 9, 2007.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
The objective of this case-control study was to evaluate whether occupational conditions during pregnancy are associated with preterm delivery (PTD). Women whose work conditions changed following the use of a legally justified preventive measure (withdrawal from work or job reassignment) were also compared with those whose work conditions did not change. Cases (n = 1,242) and controls (n = 4,513) were selected from 43,898 women who had single livebirths between January 1997 and March 1999 in Québec, Canada. They were interviewed by telephone after delivery. Results showed association of PTD with demanding posture for at least 3 hours per day, whole-body vibrations, high job strain combined with low or moderate social support, and a cumulative index composed of nine occupational conditions. The adjusted odds ratio increased from 1.0 to 2.0 for PTD (ptrend < 0.0001) and from 1.0 to 2.7 for very PTD (<34 weeks; ptrend = 0.0015) as the number of conditions increased from zero to four or more. The associations for PTD and very PTD with most of the above-mentioned work conditions were weaker when exposures were eliminated following recourse to a legally justified preventive measure. This study provides relevant information on the possible influence of preventive measures on the risk of PTD in pregnant workers.

maternal exposure; occupational exposure; posture; pregnancy outcome; premature birth; stress, psychological; vibration; work schedule tolerance

Abbreviations: PTD, preterm delivery


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
In industrialized countries, preterm delivery (PTD) is usually considered to be the most important cause of infant mortality (1–4) and a major determinant of morbidity (3–5) and neurodevelopmental deficits (1–3). Maternal risk factors for this condition include medical conditions (1, 3–9), smoking (1, 3, 6, 8), and social (1, 3, 8) and occupational (7) factors.

Previous prospective cohorts (10–24), retrospective studies (25–42), and case-control studies (43–50) have investigated the association between PTD and long working hours (12, 13, 18, 19, 21, 25, 26, 28, 31, 33–39, 47, 49), inconvenient schedule (17, 19, 23, 25, 28, 33, 34, 37, 40, 41, 46, 47, 49), prolonged standing (12–17, 19–21, 26–28, 33, 34, 36–38, 43, 46–49), lifting loads (10, 11, 16, 17, 19, 21, 25, 27, 28, 30, 32–34, 37, 38, 43, 49), high psychosocial stress (10, 11, 13, 22, 25, 26, 29, 33, 44–48, 50), or a cumulative index of occupational fatigue (13, 15, 24, 26, 30, 33, 34, 37, 38, 42, 47, 48, 50). Results were inconsistent. Increased risk of PTD was documented in 67 percent of studies on long working hours (12, 13, 18, 21, 25, 26, 31, 33, 34, 36, 47, 49), 62 percent on inconvenient schedule (17, 19, 25, 28, 33, 34, 41, 47), 68 percent on prolonged standing (12, 14–17, 20, 26, 27, 33, 36–38, 46, 47, 49), 53 percent on lifting loads (10, 17, 19, 21, 27, 30, 34, 37, 38), 71 percent on high psychosocial stress (11, 26, 29, 33, 44–48, 50), and 77 percent on cumulative fatigue score (15, 24, 30, 33, 34, 37, 42, 47, 48, 50). For individual exposures (10–23, 25–41, 43–50), most associations were of small magnitude (median value of the 84 measures of associations: 1.29). The few statistically significant associations either were greater than 1.50 (15, 16, 20, 25, 30, 31, 33, 36, 41, 47, 48), came from large sample size studies (14, 34, 49), or were related to fatigue score (30, 33, 34, 37, 42, 47, 48, 50). Results did not vary consistently with study design. Increased risks of PTD with long working hours, inconvenient schedule, and lifting loads were more frequently observed in prospective studies (10/14, 71 percent) than in retrospective and case-control studies (19/34, 56 percent), whereas the contrary was observed for psychosocial stress and fatigue score (3/7, 43 percent and 17/20, 85 percent, respectively). Several other methodological issues could explain inconsistency in study results. Previous studies with small sample size (10–13, 15–22, 24, 25, 27–33, 37, 38, 40, 41, 43, 44, 46, 47) were more likely to show associations between PTD and standing than were other studies. The same was true for low (under 80 percent) participation rate (11–13, 16, 19, 20, 23, 24, 27, 29, 32, 33, 35–37, 39, 42, 44, 45, 47–49) with standing, psychosocial stress, or fatigue score and for potential confounding (11–13, 15, 17, 18, 20–22, 24–27, 30–33, 35–38, 40, 42, 43, 46–50) with long hours or prolonged standing.

In two meta-analyses, statistically significant pooled estimates of 1.2–1.3 were found for shift or night work (51, 52), long working hours (51, 52) (in a subset of high-quality studies), prolonged standing (51, 52), and physically demanding work (52). A pooled estimate of 1.63 was reported for a cumulative work fatigue score (52). For standing and physically demanding work, pooled associations were consistent across study designs (52). Except for long working hours, pooled estimates did not vary according to study quality score (51, 52).

Studies where authors failed to take into account changes in work conditions during pregnancy, usually experienced by the most heavily exposed workers (10–12, 14, 16, 23, 30, 34, 36–38, 46, 49), are less likely to observe results associating PTD with long working hours, inconvenient schedule, and psychosocial stress than are other studies. A possible underestimation of PTD risk in women still working in late pregnancy (19, 53, 54) can result from a "healthy pregnant worker effect" if healthier women without pregnancy complications select themselves to continue working late in pregnancy.

In the province of Québec, Canada, a favorable context exists to examine how occupational condition changes occurring during pregnancy relate to pregnancy outcome. According to the Loi sur la santé et sécurité du travail (Health and Work Security Act), pregnant workers have a legal right to be assigned to other tasks or to withdraw from work without prejudice if working conditions present a danger to themselves or the fetus (55). Recourse to preventive measures is frequent (nearly half of pregnant workers) (56). It does not depend on the woman's health or on the union's or employer's willingness. The request is initiated by the woman's physician who consults a public health occupational physician. The latter formulates a recommendation after completion with the pregnant worker of a standardized evaluation of her working conditions. The pertinence of the recommendation is finally determined by the Commission de la santé et de la sécurité au travail, the governmental agency for health and safety at work.

Our objective was to estimate the association between some occupational conditions, both individually and cumulatively, and the risk of PTD. We also assessed whether the risk of PTD was lower when a woman's work conditions changed following the use of a legally justified preventive measure (withdrawal from work or job reassignment) than when they did not change.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 References
 
Study design and population
For this case-control study, the source population consisted of women living in seven regions (Saguenay-Lac-St-Jean, Québec, Mauricie, Centre-du-Québec, Estrie, Laval, Chaudière-Appalaches) of the province of Québec who gave birth to a live singleton between January 25, 1997, and March 7, 1999. Those regions are representative of urban, semiurban, and rural populations of the province. We were authorized by the Commission d'accès à l'Information du Québec to obtain the mother's name and telephone number; type of birth (single or multiple); and birth date and length of pregnancy, all recorded on birth certificates.

The length of pregnancy is usually estimated by comparing the actual date of delivery with the expected birth date, the latter determined by the physician from the date of the last menses and clinical and ultrasonic evaluations (28).

A total of 43,898 singleton livebirths were reported to us; according to government data, this number represented 94 percent of singleton livebirths in the seven participating regions during the study period. We classified births as PTD cases (infants born before 37 complete weeks of gestation) (n = 2,626 (6 percent)) or noncases (n = 41,272). A random sample of 20 percent of the noncases (n = 8,365) constituted the potential control group.

Data collection
After receipt of the birth certificate information, the interviewers contacted the women by telephone. The median interval between childbirth and interview was 32 days for cases and 30 days for controls. The interviewers explained the study, requested the woman's participation, and verified her eligibility. Among the case mothers, 192 (7.3 percent) could not be contacted, and 56 (2.1 percent) refused to participate; among the control mothers, these numbers were 470 (5.6 percent) and 128 (1.5 percent), respectively. Of the 10,145 women (2,378 cases and 7,767 controls) who agreed to participate, women who did not work (n = 3,294), those who worked less than 4 weeks from the first month of pregnancy (n = 403) or less than 20 hours per week (n = 457), and those with more than one job (n = 236) were excluded. This left 5,755 eligible women (1,273 cases and 4,482 controls) who completed a computer-assisted telephone interview of 20–30 minutes.

The questionnaire documented the following working conditions: schedule (hours and consecutive days worked per week, day, evening, or night work, schedule regularity); posture (sitting, standing, other demanding postures); physical effort (lifting (weight and frequency), pushing, and pulling objects); work organization (breaks, piecework or assembly line work, psychosocial factors); and environmental occupational conditions (e.g., whole-body vibration, temperature, exposure to environmental tobacco smoke).

We developed the questions after examining previous questionnaires (33, 34) and existing literature (57–63) and after consulting ergonomists. Psychosocial factors were evaluated by use of the job demand-control support model of Karasek and Theorell (64) and a validated French version of related scales (65, 66). Psychological demand and decision latitude scales were dichotomized at the median value. Social support by colleagues and supervisors was categorized in two levels on the basis of the natural distribution of the score. Four levels of job strain were obtained by cross-stratifying psychological demand and decision latitude (high, active, passive, low). The three highest levels of job strain were also subdivided by social support level.

For the first step, we documented working conditions at the beginning of pregnancy. If conditions were modified during pregnancy, we asked when and documented the new working conditions related to work schedule, posture, and effort. Mothers also indicated when they stopped working and why (e.g., legally justified preventive withdrawal, health problems, coming close to expected date of delivery).

The final section of the questionnaire documented obstetric history, mother's medical profile (before and during pregnancy), newborn's characteristics (gender, weight, birth date, expected date of delivery according to the physician, congenital anomalies), mother's lifestyle (drug consumption, physical activity, and consumption in third trimester of caffeine, tobacco, and alcohol), and sociodemographic characteristics.

For 154 (2.7 percent) of the 5,755 women, the interview data (date of birth and expected date of delivery) indicated a case or control status different from that determined on the basis of birth certificate data. Of these women, 115 (74.7 percent) gave us access to their hospital records to verify the information. Archivists responded to 114 (99.1 percent) of the requests. The information received resulted in an amendment of case or control status for 57 (50 percent) of these 114 subjects, resulting in 1,242 cases and 4,513 controls in the study population.

Analysis
Odds ratios and 95 percent confidence intervals were estimated from beta coefficients and their standard errors produced by logistic regression.

All variables statistically associated (p < 0.05) with PTD or with the use of preventive measures as those with an odds ratio greater than or equal to 1.2 (tables 1 and 2) were considered as potential confounders and were initially entered as covariates in all regression models. We first obtained odds ratios relating occupational conditions to PTD by multiple logistic regression with adjustment for the whole set of covariates. Covariates were withdrawn one by one as long as the odds ratio was not modified by more than 10 percent compared with the full model. In the final model, odds ratios were also adjusted for other occupational conditions present at the beginning of pregnancy (refer to footnotes of tables 3, 4, and 5).


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  		TABLE 1. Odds ratios and 95% confidence intervals for having a preterm delivery, by potential confounding variables, among workers giving birth in Québec, Canada, between January 1997 and March 1999

 

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  		TABLE 2. Odds ratios and 95% confidence intervals for using legally justified preventive measures to eliminate occupational conditions that could increase risk of preterm delivery, by sociodemographic, lifestyle, medical, and occupational variables, among workers giving birth in Québec, Canada, between January 1997 and March 1999

 

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  		TABLE 3. Adjusted odds ratios and 95% confidence intervals for preterm delivery, by occupational condition at beginning of pregnancy and early (<24 weeks), late (≥24 weeks), or no elimination of condition by legally justified preventive measures during pregnancy, among workers giving birth in Québec, Canada, between January 1997 and March 1999

 

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  		TABLE 4. Adjusted{dagger} odds ratios and 95% confidence intervals for very preterm delivery (<34 weeks), by occupational condition at beginning of pregnancy, among workers giving birth in Québec, Canada, between January 1997 and March 1999

 

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  		TABLE 5. Adjusted odds ratios and 95% confidence intervals for preterm delivery and very preterm delivery, by the proportion of workers exposed to the condition based on job title,{dagger} among workers giving birth in Québec, Canada, between January 1997 and March 1999

 
To assess the influence of changes in occupational conditions by legally justified preventive measures, we divided workers exposed at the beginning of pregnancy to a given working condition into three groups according to whether the condition was eliminated by preventive measures early (before 24 weeks) or late (at or after 24 weeks) in pregnancy or not eliminated. The PTD risks of these three groups were compared with the risk for unexposed workers at the beginning of pregnancy.

A secondary analysis based on job title was performed to assess the potential for a recall bias related to self-reported exposure. Using a 20 percent random sample of the source population, we calculated the percentage of workers reporting exposure to a given condition for each of 60 job titles. Women were classified according to the probability of exposure (percent) corresponding to their job title.

We calculated a cumulative index of occupational conditions for which the adjusted odds ratio was at least 1.2 when the condition was not eliminated by a preventive measure. The association of this index with PTD risk was adjusted for relevant covariates. We used a {chi}2 test to evaluate a dose-response relation (67). We also evaluated the influence of eliminating the indexed conditions by preventive measures during the pregnancy using the method described previously for single occupational conditions. For individual conditions and for cumulative index, adjusted odds ratios were obtained for very PTD (<34 weeks).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 References
 
Among the potential confounders, the strongest associations (odds ratio > 1.5) with PTD were observed for congenital anomalies, chronic hypertension, mother's age, and prior adverse pregnancies. Physical activity, ethnicity, caffeine and alcohol consumption, and illicit drugs during pregnancy were not associated with PTD risk (data not shown).

Nearly half (47.3 percent) of the workers used legally justified preventive measures: preventive withdrawal from work (32.3 percent), job reassignment (22.0 percent), or both (7.0 percent). Frequency use of preventive measures was similar for the cases and the controls. The number of occupational conditions was strongly related to recourse to preventive measures and was by far the most important factor explaining their use (table 2). The association was even stronger for early than for late preventive measures (data not shown). After adjustment for occupational conditions, preventive measures remained associated with low education, family income, low maternal age, chronic hypertension, diabetes, and over-the-counter drug use. All variables shown in table 1 were considered as potential confounders and were entered in the initial multivariate models. This set of covariates includes the variables associated with PTD, as well as those associated with recourse to preventive measures in table 2.

Of the occupational conditions present at the beginning of pregnancy, demanding posture (bending, squatting, arms raised above shoulder level) for at least 3 hours per day, whole-body vibrations, and high job strain combined with low or moderate social support were significantly associated with PTD (table 3). The associations with demanding posture and job strain were higher when not eliminated by preventive measures than when they were eliminated early during pregnancy. Odds ratios of at least 1.2 were observed for a few other conditions when they were not eliminated by preventive measures.

Of the three work conditions associated with PTD, whole-body vibrations and high job strain combined with low or moderate social support were related (odds ratio ≥ 1.5) to very PTD (table 4). Workers exposed to more than 5 consecutive working days were also at increased risk of very PTD. The odds ratios were 2.8, 2.1, and 1.7, respectively, when the conditions were not eliminated by preventive measures, and the first two are statistically significant (data not shown).

There was no association between PTD risk and the following conditions, whether or not the worker took recourse to preventive measures: maximum number of hours worked per week, possibility to sit when standing, pushing or pulling objects, having to climb stairs, absence of breaks, piece work or assembly line, noise, long commuting time to work, and exposure to environmental tobacco smoke at work (data not shown).

In the job title analysis, working more than 5 consecutive days, demanding posture, whole-body vibration, and high strain combined with low or moderate social support remained associated with PTD or very PTD, with odds ratios ranging from 1.3 to 2.6 (table 5).

We calculated a cumulative index, composed of the nine occupational conditions showing an odds ratio of 1.2 or greater for PTD when not eliminated by a preventive measure. These conditions are more than 5 consecutive working days, irregular or shift-work schedule, standing at least 7 hours per day mostly in one spot, sitting posture for at least 3 hours per day with rare or no possibility to stand, demanding posture for at least 3 hours per day, whole-body vibrations, very hot or very cold temperatures, and moderate-active or high job strain combined with low or moderate social support. Odds ratios increased from 1.0 to 2.0 for PTD and to 2.7 for very PTD, with significant trends, as the number of indexed conditions increased from zero to 4–6 (table 6). Trends were stronger when indexed conditions were not eliminated by preventive measures. Except for workers exposed to four or more occupational conditions (2.9 percent of workers in the source population), the PTD and very PTD risks were lower when occupational conditions were eliminated by preventive measures. Odds ratios were higher for very PTD than for PTD, whether the cumulative index calculated at the beginning of pregnancy was considered or the number of conditions not eliminated by preventive measures.


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  		TABLE 6. Odds ratios and 95% confidence intervals for preterm delivery and very preterm delivery, by cumulative index of occupational conditions{dagger} at the beginning of pregnancy and early (<24 weeks), late (≥24 weeks), or no elimination of conditions by legally justified preventive measures during pregnancy, among workers giving birth in Québec, Canada, between January 1997 and March 1999

 
The exclusion of newborns with congenital anomalies had no influence on the associations between PTD risk and individual occupational conditions or cumulative index of conditions present at the beginning of pregnancy (data not shown).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
References
 
We found that an increased risk of PTD was significantly associated with demanding posture for at least 3 hours per day, whole-body vibrations, and high job strain combined with low or moderate social support. Increased risks of very PTD were associated with the last two conditions. Risks of PTD and very PTD increased with a cumulative index composed of nine work conditions including the three above. Most of these associations were higher in women whose work conditions did not change compared with women whose work conditions changed in the course of pregnancy following a legally justified job withdrawal or reassignment.

Some earlier studies found increased PTD risk for demanding posture (21, 35, 50, 68) and vibrations (34). Ten (11, 26, 29, 33, 44–48, 50) of 14 earlier studies on psychosocial stress at work (10, 11, 13, 22, 25, 26, 29, 33, 44–48, 50) showed increased PTD risk. The association with a cumulative index of exposure is consistent with a meta-analysis (52) and with most (15, 24, 30, 33, 34, 37, 42, 47, 48, 50) previous studies that evaluated the relation between the accumulation of strenuous working conditions and PTD. Only three studies (13, 26, 38) did not find a positive trend, and the authors of one study report that preventive measures (increased antenatal leave, modifications in working conditions) could be one reason for the lack of relation (38).

Given that interviews took place after delivery, a recall bias could overestimate the associations if women who had PTD were more likely to report adverse working conditions. There is no clear evidence that recall bias was present in previous retrospective studies or case-control studies of PTD. Although retrospective (25–42) and case-control (43–50) studies were more likely than prospective ones (10–24) to result in positive associations with psychosocial stress and cumulative index of work fatigue, the contrary was observed for long working hours, inconvenient schedule, and lifting loads, and proportions were similar for prolonged standing. Nevertheless, we conducted a secondary analysis based on job title, which is less prone to recall bias than analysis based on self-reported exposure. In spite of the nondifferential misclassification induced by job title analysis and the fact that changes in work exposures during pregnancy were not taken into account, this approach confirmed the main findings of tables 3 and 4. Indeed, the three exposures with significantly elevated odds ratios for PTD (demanding posture, vibrations, and high strain) and the three with odds ratios greater than or equal to 1.5 for very PTD (>5 consecutive working days, vibrations, and high strain) remained associated with PTD or very PTD. Several occupational conditions that women may have suspected to be detrimental to their pregnancies (e.g., night work, lifting loads) were not associated with PTD risk. Finally, in order to prevent differential report, the interviewers were unaware of the mothers' case or control status when they questioned them about working conditions. Altogether, these arguments, we believe, are not in favor of a recall bias although the latter cannot totally be excluded.

The analysis by recourse to legally justified preventive measures is an interesting contribution of this study. Our results are consistent with those of studies where PTD risks were lower when pregnant women had modification of their working conditions (69) or had access to more frequent or longer antenatal leave (26, 69) and with those where higher PTD risks were present among women who continued to work later during pregnancy despite arduous working conditions (11, 28, 45, 70) or high strain (12, 45). In a study comparing European countries, significant associations of PTD with working conditions were observed only in countries where long prenatal leaves were infrequent (49).

As health conscious women are more likely to use preventive measures, one might question whether results are confounded by the woman's characteristics linked to selective implementation of preventive measures (job withdrawal or reassignment). However, the associations shown in table 2 do not support this argument. First, the number of occupational exposures is the factor most strongly related to the use of preventive measures. Second, after adjustment for work conditions, recourse to preventive measures remains more frequent in the youngest and less educated women and in those with moderate or low family income, as well as in those suffering from chronic hypertension or diabetes, and is not affected by lifestyle. Therefore, it seems unlikely, although still possible, that personal characteristics linked to a better pregnancy outcome increase the likelihood of requesting preventive measures and explain the suggested benefit of preventive measures.

All variables linked to PTD or to recourse to preventive measures were taken into account in the analyses leading to tables 3–6. Pregnancy complications (e.g., bleeding, gestational hypertension) were not included as covariates because they can be intermediate factors in the causal pathway linking occupational conditions to PTD. Nevertheless, the observed associations are weak, and the possibility of residual confounding cannot be eliminated.

To minimize misclassification of outcomes, we cross-checked the case or control status established on the basis of birth certificates with the status derived from interview. Discordance was rare, and information could be checked against hospital records for 74 percent of these newborns. The high participation rate and the large number of potential confounders considered in the analyses are additional characteristics that enhance the validity of our results.

In conclusion, our results show higher PTD risk for pregnant working women exposed to demanding posture for at least 3 hours per day, whole-body vibrations, high job strain combined with low or moderate social support, and a cumulative index composed of nine work conditions. Women who experienced a change in these work conditions following recourse to preventive job withdrawal or reassignment had lower risks of PTD and very PTD than those who did not. Although the associations are small, most of them are consistent for PTD and very PTD. A secondary analysis based on job title supports the view that recall bias does not explain the results. As experimental studies to assess changes in work conditions during pregnancy are not likely to be carried out for ethical and feasibility considerations, our observational study despite its limitations provides relevant information on this question.


   ACKNOWLEDGMENTS
 
Funding for this study was provided by Health Canada under the National Health Research and Development Program. The study was conducted with the support of the Quebec Regional Public Health Direction. C. Brisson holds a research scientist award from the Canadian Institutes of Health Research.

The authors thank Dr. S. Montreuil, Dr. L. Punnett, and Dr. L. Patry, ergonomists, for their scientific input in questionnaire development; G. Bergeron for assistance in data processing; and M. Desgagné, S. Mercier, and C. Pelletier for conducting telephone interviews. The authors would also like to thank the Regional Public Health Directions.

Conflict of interest: none declared.


   References
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 

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