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Preterm Delivery and Work

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The reconciliation of work and maternity is an important public health issue in industrialized countries, where more than 50% of women of child-bearing age work outside the home. Working women, unions, employers, politicians and clinicians are all searching for ways of preventing work-induced unfavourable reproductive outcomes. Women want to continue working while pregnant, and may even consider their physician’s advice about lifestyle modifications during pregnancy to be overprotective and unnecessarily restrictive.

physiological Consequences of pregnancy

At this point, it would be useful to review a few of the physiological consequences of pregnancy that may interfere with work.

A pregnant woman undergoes profound changes which allow her to adapt to the needs of the foetus. Most of these changes involve the modification of physiological functions that are sensitive to changes of posture or physical activity—the circulatory system, the respiratory system and water balance. As a result, physically active pregnant women may experience unique physiological and physiopathological reactions.

The main physiological, anatomical, and functional modifications undergone by pregnant women are (Mamelle et al. 1982):

  1. An increase in peripheral oxygen demand, leading to modification of the respiratory and circulatory systems. Tidal volume begins to increase in the third month and may amount to 40% of re-pregnancy values by the end of the pregnancy. The resultant increase in gas exchange may increase the hazard of the inhalation of toxic volatiles, while hyperventilation related to increased tidal volume may cause shortness of breath on exertion.
  2. Cardiac output increases from the very beginning of pregnancy, as a result of an increase in blood volume. This reduces the heart’s ability to adapt to exertion and also increases venous pressure in the lower limbs, rendering standing for long periods difficult.
  3. Anatomical modifications during pregnancy, including exaggeration of dorsolumbar lordosis, enlargement of the polygon of support and increases in abdominal volume, affect static activities.
  4. A variety of other functional modifications occur during pregnancy. Nausea and vomiting result in fatigue; daytime sleepiness results in inattention; mood changes and feelings of anxiety may lead to interpersonal conflicts.
  5. Finally, it is interesting to note that the daily energy requirements during pregnancy are equivalent to the requirements of two to four hours of work.


Because of these profound changes, occupational exposures may have special consequences in pregnant women and may result in unfavourable pregnancy outcomes.

Epidemiological Studies of Working Conditions and preterm Delivery

Although there are many possible unfavourable pregnancy outcomes, we review here the data on preterm delivery, defined as the birth of a child before the 37th week of gestation. preterm birth is associated with low birth weight and with significant complications for the newborn. It remains a major public health concern and is an ongoing reoccupation among obstetricians.

When we began research in this field in the mid-1980s, there was relatively strong legislative protection of pregnant women’s health in France, with prenatal maternity leave mandated to start six weeks prior to the due date. Although the preterm delivery rate has fallen from 10 to 7% since then, it appeared to have levelled off. Because medical prevention had apparently reached the limit of its powers, we investigated risk factors likely to be amenable to social intervention. Our hypotheses were as follows:

    • Is working per se a risk factor for preterm birth?
    • Are certain occupations associated with an increased risk of preterm delivery?
    • Do certain working conditions constitute a hazard to the pregnant woman and foetus?
    • Are there social preventive measures which could help reduce the risk of preterm birth?


          Our first study, conducted in 1977–78 in two hospital maternity wards, examined 3,400 women, of whom 1,900 worked during pregnancy and 1,500 remained at home (Mamelle, Laumon and Lazar 1984). The women were interviewed immediately after delivery and asked to describe their home and work lifestyle during pregnancy as accurately as possible.

          We obtained the following results:

          Work per se

          The mere fact of working outside the home cannot be considered to be a risk factor for preterm delivery, since women remaining at home exhibited a higher prematurely rate than did women who worked outside the home (7.2 versus 5.8%).

          Working conditions

          An excessively long work week appears to be a risk factor, since there was a regular increase in preterm delivery rate with the number of work hours. Retail-sector workers, medical social workers, specialized workers and service personnel were at higher risk of preterm delivery than were office workers, teachers, management, skilled workers or supervisors. The prematurely rates in the two groups were 8.3 and 3.8% respectively.

          Table 1. Identified sources of occupational fatigue

          Occupational fatigue index “HIGH” index if:
          Posture Standing for more than 3 hours per day
          Work on machines Work on industrial conveyor belts; independent work on industrial machines with strenuous effort
          Physical load Continuous or periodical physical effort; carrying loads of more than 10kg
          Mental load Routine work; varied tasks requiring little attention without stimulation
          Environment Significant noise level; cold temperature; very wet atmosphere; handling of chemical substances

          Source: Mamelle, Laumon and Lazar 1984.

          Task analysis allowed identification of five sources of occupation fatigue: posture, work with industrial machines, physical workload, mental workload and the work environment. Each of these sources of occupational fatigue constitutes a risk factor for preterm delivery (see tables 1 and 2).

          Table 2. Relative risks (RR) and fatigue indices for preterm delivery

          Index Low index % High index % RR Statistical significance
          Posture 4.5 7.2 1.6 Significant
          Work on machines 5.6 8.8 1.6 Significant
          Physical load 4.1 7.5 1.8 Highly significant
          Mental load 4.0 7.8 2.0 Highly significant
          Environment 4.9 9.4 1.9 Highly significant

          Source: Mamelle, Laumon and Lazar 1984.

          Exposure to multiple sources of fatigue may result in unfavourable pregnancy outcomes, as evidenced by the significant increase of the rate of preterm delivery with an increased number of sources of fatigue (table 3). Thus, 20% of women had concomitant exposure to at least three sources of fatigue, and experienced a preterm delivery rate twice as high as other women. Occupational fatigue and excessively long work weeks exert cumulative effects, such that women who experience intense fatigue during long work weeks exhibit an even higher prematurely rate. preterm delivery rates increase further if the woman also has a medical risk factor. The detection of occupational fatigue is therefore even more important than the detection of medical risk factors.

          Table 3. Relative risk of prematurity according to number of occupational fatigue indices

          Number of high
          fatigue indices
          Proportion of
          exposed women %
          relative risk
          0 24 1.0
          1 28 2.2
          2 25 2.4
          3 15 4.1
          4-5 8 4.8

          Source: Mamelle, Laumon and Lazar 1984

          European and North American studies have confirmed our results, and our fatigue scale has been shown to be reproducible in other surveys and countries.

          In a case-control follow-u study conducted in France a few years later in the same maternity wards (Mamelle and Munoz 1987) , only two of the five previously defined indices of fatigue were significantly related to preterm delivery. It should however be noted that women had a greater opportunity to sit down and were withdrawn from physically demanding tasks as a result of preventive measures implemented in the workplaces during this period. The fatigue scale nevertheless remained a predictor of preterm delivery in this second study.

          In a study in Montreal, Quebec (McDonald et al. 1988), 22,000 pregnant women were interviewed retrospectively about their working conditions. Long work weeks, alternating shift work and carrying heavy loads were all shown to exert significant effects. The other factors studied did not appear to be related to preterm delivery, although there appears to be a significant association between preterm delivery and a fatigue scale based on the total number of sources of fatigue.

          With the exception of work with industrial machines, no significant association between working conditions and preterm delivery was found in a French retrospective study of a representative sample of 5,000 pregnant women (Saurel-Cubizolles and Kaminski 1987). However, a fatigue scale inspired by our own was found to be significantly associated with preterm delivery.

          In the United States, Homer, Beredford and James (1990), in a historical cohort study, confirmed the association between physical workload and an increased risk of preterm delivery. Teitelman and co-workers (1990), in a prospective study of 1,200 pregnant women, whose work was classified as sedentary, active or standing, on the basis of job description, demonstrated an association between work in a standing position and preterm delivery.

          Barbara Luke and co-workers (in press) conducted a retrospective study of US nurses who worked during pregnancy. Using our occupational risk scale, she obtained similar results to ours, that is, an association between preterm delivery and long work weeks, standing work, heavy workload and unfavourable work environment. In addition, the risk of preterm delivery was significantly higher among women with concomitant exposure to three or four sources of fatigue. It should be noted that this study included over half of all nurses in the United States.

          Contradictory results have however been reported. These may be due to small sample sizes (Berkowitz 1981), different definitions of prematurely (Launer et al. 1990) and classification of working conditions on the basis of job description rather than actual workstation analysis (Klebanoff, Shiono and Carey 1990). In some cases, workstations have been characterized on a theoretical basis only—by the occupational physician, for example, rather than by the women themselves (peoples-Shes et al. 1991). We feel that it is important to take subjective fatigue—that is, fatigue as it is described and experienced by women—into account in the studies.

          Finally, it is possible that the negative results are related to the implementation of preventive measures. This was the case in the prospective study of Ahlborg, Bodin and Hogstedt (1990), in which 3,900 active Swedish women completed a self-administered questionnaire at their first prenatal visit. The only reported risk factor for preterm delivery was carrying loads weighing more than 12 kg more often than 50 times per week, and even then the relative risk of 1.7 was not significant. Ahlborg himself points out that preventive measures in the form of aid maternity leave and the right to perform less tiring work during the two months receding their due date had been implemented for pregnant women engaged in tiring work. Maternity leaves were five times as frequent among women who described their work as tiring and involving the carrying of heavy loads. Ahlborg concludes that the risk of preterm delivery may have been minimized by these preventive measures.

          preventive Interventions: French Examples

          Are the results of aetiological studies convincing enough for preventive interventions to be applied and evaluated? The first question which must be answered is whether there is a public health justification for the application of social preventive measures designed to reduce the rate of preterm delivery.

          Using data from our previous studies, we have estimated the proportion of preterm births caused by occupational factors. Assuming a rate of preterm delivery of 10% in populations exposed to intense fatigue and a rate of 4.5% in non-exposed populations, we estimate that 21% of premature births are caused by occupational factors. Reducing occupational fatigue could therefore result in the elimination of one-fifth of all preterm births in French working women. This is ample justification for the implementation of social preventive measures.

          What preventive measures can be applied? The results of all the studies lead to the conclusion that working hours can be reduced, fatigue can be lessened through workstation modification, work breaks can be allowed and prenatal leave can be lengthened. Three cost-equivalent alternatives are available:

            • reducing the work week to 30 hours starting from the 20th week of gestation
            • prescribing a work break of one week each month starting in the 20th week of gestation
            • beginning prenatal leave at the 28th week of gestation.


                It is relevant to recall here that French legislation provides the following preventive measures for pregnant women:

                  • guaranteed employment after childbirth
                  • reduction of the workday by 30 to 60 minutes, applied through collective agreements
                  • workstation modification in cases of incompatibility with pregnancy
                  • work breaks during pregnancy, prescribed by attending physicians
                  • prenatal maternity leave six weeks prior to the due date, with a further two weeks available in case of complications
                  • postnatal maternity leave of ten weeks.


                            A one-year prospective observational study of 23,000 women employed in 50 companies in the Rhône-Ales region of France (Bertucat, Mamelle and Munoz 1987) examined the effect of tiring work conditions on preterm delivery. Over the period of the study, 1,150 babies were born to the study population. We analysed the modifications of working conditions to accommodate pregnancy and the relation of these modifications to preterm delivery (Mamelle, Bertucat and Munoz 1989), and observed that:

                              • Workstation modification was reformed for only 8% of women.
                              • 33% of women worked their normal shifts, with the others having their workday reduced by 30 to 60 minutes.
                              • 50% of women took at least one work break, apart from their prenatal maternity leave; fatigue was the cause in one-third of cases.
                              • 90% of women stopped working before their legal maternity leave began and obtained at least the two weeks leave allowed for in the case of complications of pregnancy; fatigue was the cause in half the cases.
                              • In all, given the legal prenatal leave period of six weeks prior to the due date (with an additional two weeks available in some cases), the real duration of prenatal maternity leave was 12 weeks in this population of women subjected to tiring work conditions.


                                      Do these modifications of work have any effect on the outcome of pregnancy? Workstation modification and the slight reduction of the workday (30 to 60 min) were both associated with non-significant reductions of the risk of preterm delivery. We believe that further reductions of the work week would have a greater effect (table 4).

                                      Table 4. Relative risks of prematurity associated with modifications in working conditions

                                      in working
                                      Number of women Preterm
                                      birth rates
                                      Relative risk
                                      (95% confidence intervals)
                                      Change in work situation
                                      0.5 (0.2-1.6)
                                      Reduction of weekly working hours
                                      0.7 (0.4-1.1)
                                      Episodes of sick leave1
                                      0.4 (0.2-0.7)
                                      Increase of antenatal maternity leave1
                                      None or only additional 2 weeks


                                      1.7 (0.9-3.0)

                                      1 In a reduced sample of 778 women with no previous or present obstetric pathology.

                                      Source: Mamelle, Bertucat and Munoz 1989.


                                      To analyse the relation between prenatal leave, work breaks and preterm delivery, it is necessary to discriminate between preventive and curative work breaks. This requires restriction of the analysis to women with uncomplicated pregnancies. Our analysis of this subgroup revealed a reduction of the preterm delivery rate among women who took work breaks during their pregnancy, but not in those who took prolonged prenatal leave (Table 9).

                                      This observational study demonstrated that women who work in tiring conditions take more work breaks during their pregnancies than do other women, and that these breaks, particularly when motivated by intense fatigue, are associated with reductions of the risk of preterm delivery (Mamelle, Bertucat and Munoz 1989).

                                      Choice of preventive Strategies in France

                                      As epidemiologists, we would like to see these observations verified by experimental preventive studies. We must however ask ourselves which is more reasonable: to wait for such studies or to recommend social measures aimed at preventing preterm delivery now?

                                      The French Government recently decided to include a “work and pregnancy guide”, identical to our fatigue scale, in each pregnant woman’s medical record. Women can thus calculate their fatigue score for themselves. If work conditions are arduous, they may ask the occupational physician or the person responsible for occupational safety in their company to implement modifications aimed at alleviating their workload. Should this be refused, they can ask their attending physician to prescribe rest weeks during their pregnancy, and even to prolong their prenatal maternity leave.

                                      The challenge is now to identify preventive strategies that are well adapted to legislation and social conditions in every country. This requires a health economics approach to the evaluation and comparison of preventive strategies. Before any preventive measure can be considered generally applicable, many factors have to be taken into consideration. These include effectiveness, of course, but also low cost to the social security system, resultant job creation, women’s references and the acceptability to employers and unions.

                                      This type of problem can be resolved using multicriteria methods such as the Electra method. These methods allow both the classification of preventive strategies on the basis of each of a series of criteria, and the weighting of the criteria on the basis of political considerations. Special importance can thus be given to low cost to the social security system or to the ability of women to choose, for example (Mamelle et al. 1986). While the strategies recommended by these methods vary depending on the decision makers and political options, effectiveness is always maintained from the public health standpoint.



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