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Occupational Medicine Advance Access published online on June 12, 2007
Occupational Medicine, doi:10.1093/occmed/kqm038
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© The Author 2007. Published by Oxford University Press on behalf of the Society of Occupational Medicine. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Maternal occupation and adverse pregnancy outcomes: a Finnish population-based study

Parvez Ahmed1 and Jouni J. K. Jaakkola1,2

1 Institute of Occupational and Environmental Medicine, University of Birmingham, Birmingham, UK
2 Environmental Epidemiology Unit, Department of Public Health, University of Helsinki, Helsinki, Finland

Correspondence to: Jouni J. K. Jaakkola, Institute of Occupational and Environmental Medicine, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. Tel: +44 (0) 121 414 6671; fax: +44 (0) 121 414 6217; e-mail: j.jaakkola{at}bham.ac.uk


   Abstract
Top
 Abstract
Introduction
 Methods
 Results
 Discussion
 Conflicts of interest
 References
 
Background There is accumulating evidence that the type of work and environmental exposures in the working environment may have adverse effects on foetal development.

Aim To compare the risk of low birth weight (LBW), small-for-gestational age (SGA) and pre-term delivery (PD) in broad categories of maternal occupation including farming and forestry; factory, mining and construction; office, non-manual and service work and housewives.

Methods The study population consisted of 2568 singleton newborns of women who participated in The Finnish Prenatal Environment and Health Study after the delivery (response rate 94%). Information on maternal occupation and work during pregnancy was collected after the delivery. The health outcomes were LBW (<2500 g), SGA and PD (<37 weeks).

Results In newborns of women working in factories, mining and construction, the risk of LBW (adjusted odds ratio [OR] 3.66, 95% confidence interval [CI] 1.15–11.62), SGA (adjusted OR 1.53, 95% CI 0.73–3.21), but not the risk of PD (adjusted OR 0.64, 95% CI 0.19–2.22), was higher compared with newborns of housewives. In newborns of farmers and forestry workers, the risks of PD (adjusted OR 2.38, 95% CI 1.01–5.65), LBW (adjusted OR 2.86, 95% CI 0.78–11.58) and SGA (adjusted OR 1.51, 95% CI 0.62–3.65) were all elevated. In office, non-manual and service workers, the corresponding estimates were lower (LBW: 1.62, 0.67–3.95; SGA: 1.45, 0.92–2.28; PD: 1.18, 0.69–2.01).

Conclusions There were substantial differences in the risk of adverse pregnancy outcomes between the main branches of industry.

Keywords      Low birth weight; occupational exposure; pre-term delivery; small-for-gestational age; type of works


   Introduction
Top
 Abstract
 Introduction
Methods
 Results
 Discussion
 Conflicts of interest
 References
 
The proportion of women employed during pregnancy has increased constantly during the previous four decades, and women are working in a broader range of occupations than before. There is accumulating evidence that the type of work and environmental exposures in the working environment may have adverse effects on foetal development [1,2]. A large number of physical, chemical and psychosocial factors as well as physical load occurring in the workplace have been found or suggested to increase the risk of adverse pregnancy outcomes including spontaneous abortion, pre-term delivery (PD), low birth weight (LBW), birth defects and still birth [1–3]. There is also evidence that occupational stress may harm foetal development [4]. There are several occupations where the risk of adverse pregnancy outcomes has been found to be elevated, including chemical [5], electronic [6] and shoe industry workers [7], hairdressers [8], laboratory technicians [9], dry cleaners [10] and agricultural workers [11].

Two general research approaches are needed to identify the harmful occupational factors. First, the identification of specific compounds and their sources and levels in the workplaces is central for both short-term and long-term prevention of adverse reproductive effects. Working environments often comprise complex sets of factors influencing foetal development and therefore the identification of singular specific causal agents may be difficult. A complementary approach is the identification of the type of industries and occupations where there is an excess risk of adverse pregnancy outcomes. Focus on workers in these occupational groups can then lead to recognition of one or several factors, which may have independent or synergistic adverse effects on the foetus.

The objective of the present study was to assess the risk of LBW, small-for-gestational age (SGA) and PD in broad categories of maternal occupation including farming and forestry; factory, mining and construction; office, non-manual and service work and housewives. We compared the risks both directly and by taking into account a number of known or suggested determinants as potential confounders.


   Methods
Top
 Abstract
 Introduction
 Methods
Results
 Discussion
 Conflicts of interest
 References
 
The Finnish Prenatal Environment and Health Study recruited from a source population of all the 2751 children born from 1 May 1996 to 30 April 1997 in two geographically defined hospital districts in south-eastern Finland (Kymi and Porvoo Hospital Districts) [12]. All mothers were asked after delivery to fill in a self-administered questionnaire ‘Environment and Pregnancy’.

The Environment and Pregnancy questionnaire administered at the birth clinic inquired about maternal health in general and during pregnancy, parents' education, profession, behavioural factors, such as smoking and exercise, and details of the home and other environment. We requested the records of these women from the maternity health clinics in the two districts. Additional information on the child's birth weight, gestational age and maternal smoking habits during pregnancy were obtained from the Finnish Medical Birth Registry established in 1987 and run by National Research and Development Centre (STAKES). The study protocol was approved by the Ethical Committees of the Department of Public Health, University of Helsinki, three participating hospitals and the Ministry of Social Affairs and Health.

The primary health outcomes were foetal growth and duration of pregnancy. We used two different measures of foetal growth: LBW (<2500 g) and SGA. LBW is a traditional measure of foetal growth, which does not take into account the duration of pregnancy. SGA reflects intrauterine growth taking into account both birth weight and gestational age. Small-for-gestational-age was defined as birth weight in the lowest 10th percentile according to the week of gestation, which was calculated from the present source population of non-smoking women. The PD was defined as the length of gestation <37 weeks. We retrieved information on gestational age from maternity health clinic records. Gestational age was practically always verified by ultrasound examination during the 18th week of gestation.

We asked maternal occupational group in seven categories including (i) farming and forestry; (ii) factory, mining and construction; (iii) student; (iv) housewife; (v) unemployed; (vi) retired and (vii) office, non-manual and service work. Housewife was used as the reference category when the risks of health outcomes were compared. They represented mothers whose newborns had the lowest risk of adverse pregnancy outcomes. Although housewives differ from women working during pregnancy, their pregnancy provides the best scenario for desired foetal development. Adjustment for potential determinants of the studied pregnancy outcomes allows rough inference on the role of working conditions on foetal development.

We used the following potential confounders as the core covariates in the analyses: gender, parity, maternal age, single parenthood, regular smoking during pregnancy, alcohol consumption during pregnancy and exposure to environmental tobacco smoke. In addition, we adjusted for a combined index of maternal and paternal education as an indicator of socio-economic status (low: neither parent with vocational education; medium: either one or both parents with vocational school as highest education; high: either one or both parents with college or university education as a reference category) and paternal occupation group categorized similarly to maternal occupation.

We estimated the prevalences (%) of the reproductive outcomes with 95% confidence intervals (CIs) based on the binomial distribution. First, we compared the risks of LBW, SGA and PD in the different categories of maternal occupation. Odds ratio (OR) was the measure of association. We used logistic regression analysis to estimate adjusted ORs. First, we adjusted for the core covariates listed above. Second, we added separately family socio-economic index and paternal occupational group to the model.


   Results
Top
 Abstract
 Introduction
 Methods
 Results
Discussion
 Conflicts of interest
 References
 
The response rate in the survey was 94% and we received 97% of the records from the maternity health clinics. The respondents, 2591 mothers, had 2568 singletons and 23 twin pairs. The present study focused on 2568 singleton newborns of women.

Table 1 displays the characteristics of the study population according to the maternal occupational groups. Information on maternal occupations was missing for 30 (1.2%) mothers who were included as a separate group in the comparison. There was substantial variation in the distribution of known or putative determinants of the studied pregnancy outcomes, which reflects the differences in the source population but is partly explained by small numbers in some of the compared groups. For example, the prevalence of multiparous mothers varied from 28% in students to 90% in housewives and the corresponding range for smoking in pregnancy was from 11% in farming and forestry to 26% in unemployed (excluding 3/8 retired).


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  		Table 1. Characteristics of the study population according to maternal occupation (N = 2568)

 
The prevalence of LBW <2500 g was lowest (1.5%) among the newborns of housewives, highest among the newborns of women in factory, mining and construction (6%) and second highest among the newborns of women in farming and forestry (5%), as shown in Table 2. Table 3 shows crude and adjusted ORs of LBW with adjustment for the core covariates with and without family socio-economic status or parental occupational group. The crude OR for factory, mining and construction was 4.32 (95% CI 1.53–12.19) and the adjusted OR was 3.66 (95% CI 1.15–11.62). The additional adjustment for family socio-economic factors slightly increased at 4.09 (95% CI 1.23–13.58) and for paternal occupation decreased to 3.37 (95% CI 1.05–10.80). The crude OR for farming and forestry was 3.35 (95% CI 0.98–11.39) and adjustment for core covariates (OR 2.86, 95% CI 0.78–11.58) and additional covariates reduced the point estimate. The adjusted ORs for the newborns of unemployed (1.83, 95% CI 0.68–4.97) and office, non-manual and service workers (1.62, 95% CI 0.67–3.95) were elevated in contrast to the newborns of housewives as the reference category.


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  		Table 2. Maternal occupational group and the risk of adverse birth outcomes (N = 2568)

 

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  		Table 3. Maternal occupational group and LBW <2500 g (N = 2568)

 
The prevalence of SGA was lowest among the infants of housewives (6%). The prevalences among the newborns of unemployed (13%), mothers working in factory, mining and construction (12%), office, non-manual and service work (10%) and farming and forestry (9%) were substantially higher, as shown in Table 2. As shown in Table 4, the crude OR for the unemployed was 2.38 (95% CI 1.46–3.87), which was reduced substantially in the adjustment for the core covariates (adjusted OR 1.49, 95% CI 0.88–2.00). Further adjustment for family socio-economic status (1.63) or paternal occupations (1.46) had little influence on the estimates. The crude OR for factory, mining and construction was 2.18 (95% CI 1.10–4.33), which was also reduced when adjusted for the core covariates with an adjusted OR of 1.53 (95% CI 0.73–3.21). Additional adjustment had little influence. The crude OR for farming and forestry (1.59, 0.67–3.76) stayed relatively stable when adjusting for the core covariates (1.51, 0.62–3.65).


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  		Table 4. Maternal occupational group and SGA (N = 2568)

 
The prevalence of PD was similar among the newborns of housewives, mothers working in factory, mining and construction, office, non-manual and service work, students and unemployed between 4 and 5% (see Table 2). The prevalence was substantially higher, 11% for the newborns of mothers working in farming and forestry. The crude OR for farming and forestry was 2.72 (95% CI 1.21–6.12), as shown in Table 5. The OR adjusted for the core covariates was 2.38 (95% CI 1.01–5.65) and slightly lower when adjusted for family socio-economic factors (1.81) or paternal occupation (2.09). The ORs for other occupational groups varied from 0.60 to 1.19.


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  		Table 5. Maternal occupational group and PD (N = 2568)

 

   Discussion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
Conflicts of interest
 References
 
The results of our population-based study show a large variation in the prevalence of LBW between the studied broad categories of maternal occupation from 1.5% among the newborns of housewives to 7% among the newborns of mothers working in factory, mining and construction. The corresponding range for SGA was from 6 to 12%. There were also substantial differences in the known or proposed determinants of foetal growth including maternal age, parity, smoking, alcohol consumption and exposure to environmental tobacco smoke between the occupational groups. After adjustment for these factors, the risk of LBW was still substantially higher among the newborns of women working in factory, mining and construction and farming and forestry compared with the newborns of housewives. Also the ORs for unemployed and for office, non-manual and service work were elevated. The prevalence of PD was 4% for newborns of housewives and similar for other groups except the newborns of mother in farming and forestry whose prevalence was 11% and there was over two-fold risk of PD after adjustment for confounding.

The source population included all children born in the designated geographic region during 12 months in 1996–97. A high proportion of women participated in the study (94%). The population-based approach of the study and high participation rate minimized the potential influence of selection bias.

The outcome information was based on measurements of birth weight after deliveries, conducted in a standardized manner, and estimated duration of pregnancy, in most cases based on definition of gestational age using ultrasound. Birth weight and gestational age information was verified from two sources, study project data collection and from the Finnish Birth Registry data. SGA was based on the birth weight distributions of the non-smoking women in the source population.

We asked the participants to classify their occupation into seven groups. Most of the women provided this information, while the information was missing only for 30 women (1.2%). Misclassification of occupational group was possible, but there is no reason to believe this would have been systematic, i.e. related to the studied health outcomes.

Use of medical records and questionnaire information allowed us to take into account a large number of potential confounders. We adjusted for confounding in two stages. First, we adjusted for seven known or suspected determinants of the studied pregnancy outcomes (gender, parity, maternal age, single parenthood, regular smoking during pregnancy, alcohol consumption during pregnancy and exposure to environmental tobacco smoke). We then adjusted additionally for two constructs describing socio-economic conditions: family socio-economic index based on both parents' education and paternal occupational group. Both of these variables are strongly associated with maternal occupational group and therefore their inclusion in the model may lead to underestimation of the role of maternal occupational group as the determinant of adverse pregnancy outcomes.

Our finding of a higher risk of LBW and SGA babies among women working in factory, mining and construction compared with babies of housewives could be partly explained by relatively higher occurrence of known or putative causes of intrauterine growth limitation, such as heavy metals (lead, mercury, cadmium and nickel), chemicals (organic solvents such as toluene, aromatic and aliphatic hydrocarbons, trichloroethylene, tetrachloroethylene, glycol ethers, petrochemicals and formaldehyde) and radiation, as well as heavier physical work [2]. This finding is also consistent with a series of epidemiologic studies showing an increased risk of LBW among newborns of women in different industries including chemical [5], electronic [6] and shoe industry [7]. The risk of LBW and SGA was also elevated among newborns of women in farming and forestry. This could also be related to heavy physical load, such as lifting heavy weights, and awkward physical postures shown to be related to the risk of LBW [2]. The elevated risk of LBW and SGA in newborns of office, non-manual and service workers could be explained by various chemical, physical, ergonomic and psychosocial factors, which may differ substantially within this category. For example, exposure to solvents and environmental tobacco smoke is likely to be more common among these women than among housewives used as the reference. Interestingly, the risk of LBW and SGA was higher among the newborns of unemployed and retired compared with the newborns of housewives. The number of retired mothers was very small and thus the evidence limited.

The risk of PD was clearly highest among the newborns of women in farming and forestry, whereas the risk in the other groups, including factory, mining and construction, was similar. This could be explained by heavier workload and more awkward postures, which have been shown to increase the risk of PD [13]. There is also epidemiologic evidence that exposure to pesticides typical for some agricultural jobs increases the risk of spontaneous abortions and PD [11,14].

Our results indicate that there are substantial differences in the risk of adverse pregnancy outcomes between the main branches of industry in Finland, which are not likely to be explained by known or proposed confounders. Identification of the specific work-related causes and their distribution in different types of work is needed.


Key points

  • Our population-based study in Finland shows a large variation in the prevalence of adverse pregnancy outcomes between the studied broad categories of maternal occupation.
  • The risk of LBW was substantially higher among newborns of women working in factory, mining and construction (6%) and farming and forestry (5%) compared with newborns of housewives (1.5%) with an adjusted OR of 3.66 (1.15–11.62) and 2.86 (0.78–11.58), respectively.
  • The prevalence of PD was 4% for newborns of housewives and similar for other groups except newborns of mother in farming and forestry whose prevalence was 11%. The corresponding adjusted OR was 2.38 (95% CI 1.01–5.65).

 


   Conflicts of interest
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conflicts of interest
References
 
None declared.


   Acknowledgements
 
We thank Niina Jaakkola for her contributions to data collection and management. We are grateful to the physicians and nurses in the Kymi and Porvoo Hospital Districts for their help and support that made this study possible. This study was supported by the Ministry of Social Affairs and Health, Finland and the West Midlands Levy Board, UK.


   References
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conflicts of interest
 References
 

  1. Burdorf A, Figà-Talamanca I, Jensen TK, Thulstrup AM. Effects of occupational exposure on the reproductive system: core evidence and practical implications. Occup Med (Lond) (2006) 56:516–520.[CrossRef][Medline]

  2. Figà-Talamanca I. Occupational risk factors and reproductive health of women. Occup Med (Lond) (2006) 56:521–531.[CrossRef][Medline]

  3. Thulstrup AM, Bonde JP. Maternal occupational exposure and risk of specific birth defects. Occup Med (Lond) (2006) 56:532–543.[CrossRef][Medline]

  4. Fenster L, Schaefer C, Mathur A, et al. Psychologic stress in the workplace and spontaneous abortion. Am J Epidemiol (1995) 142:1176–1183.[Abstract/Free Full Text]

  5. Ha E, Cho SI, Chen D, et al. Parental exposure to organic solvents and reduced birth weight. Arch Environ Health (2002) 57:207–214.[Web of Science][Medline]

  6. Takeuchi Y, Ichihara G, Kamijina M. A review of toxicity of 2-promopropane: mainly on its reproductive toxicity. J Occup Health (1997) 39:179–191.[Web of Science]

  7. Agnesi R, Valentini F, Mastrangelo G. Risk of spontaneous abortion and maternal exposure to organic solvents in the shoe industry. Int Arch Occup Environ Health (1997) 69:311–316.[CrossRef][Web of Science][Medline]

  8. Kersemaekers WM, Roeleveld N, Zielhuis GA. Reproductive disorders among hairdressers. Epidemiology (1997) 8:396–401.[CrossRef][Web of Science][Medline]

  9. Zhu JL, Knudsen LE, Andersen A-MN, et al. Laboratory work and pregnancy outcomes: a study within the National Birth Cohort in Denmark. Occup Environ Med (2006) 63:53–58.[Abstract/Free Full Text]

  10. Doyle P, Roman E, Beral V, et al. Spontaneous abortion in dry cleaning workers potentially exposed to perchloroethylene. Occup Environ Med (1997) 54:848–853.[Abstract/Free Full Text]

  11. Nurminen T. Maternal pesticide exposure and pregnancy outcome. J Occup Med (1995) 37:935–940.[CrossRef][Web of Science]

  12. Jaakkola JJK, Jaakkola N, Zahlsen K. Fetal growth and length of gestation in relation to exposure to environmental tobacco smoke measured by hair nicotine concentration. Environ Health Perspect (2001) 109:557–561.[Web of Science][Medline]

  13. Escriba-Aguir V, Sandiago PH, Saurel MJ. Physical load and psychological demand at work during pregnancy and preterm birth. Int Arch Occup Environ Health (2001) 74:583–588.[Web of Science][Medline]

  14. Restrepo M, Munoz N, Day NE, et al. Prevalence of adverse reproductive outcomes in a population of occupationally exposed to pesticides in Colombia. Scand J Work Environ Health (1990) 16:232–238.[Web of Science][Medline]


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