ORIGINAL ARTICLE Occupational lifting of heavy loads and preterm birth: a study within the Danish National Birth Cohort Stine Bjerrum Runge, 1 Jacob Krabbe Pedersen, 2 Susanne Wulff Svendsen, 3 Mette Juhl, 1 Jens Peter Bonde, 4 Anne-Marie Nybo Andersen 1 1 Section of Social Medicine, Department of Public Health, University of Copenhagen, Copenhagen K, Denmark 2 Division of Epidemiology, Institute of Public Health, University of Southern Denmark, Odense, Denmark 3 Department of Occupational Medicine, Danish Ramazzini Center, Herning Hospital, Herning, Denmark 4 Department of Occupational and Environmental Medicine, Bispebjerg University Hospital, Copenhagen NV, Denmark Correspondence to Professor Anne-Marie Nybo Andersen, Section of Social Medicine, Department of Public Health, University of Copenhagen, Oster Farimagsgade 5, Box 2099, Copenhagen K DK-1014, Denmark; amny@sund.ku.dk Published Online First 9 July 2013 To cite: Runge SB, Pedersen JK, Svendsen SW, et al. Occup Environ Med 2013;70:782–788. ABSTRACT Objectives To examine the association between occupational lifting during pregnancy and preterm birth. The risk of preterm birth was estimated for total burden lifted per day and number of medium and heavy loads lifted per day. Methods In a study population of 62 803 pregnant women enrolled to the Danish National Birth Cohort from 1996 to 2002, the association between self- reported occupational lifting in the first part of pregnancy and preterm birth was analysed using logistic regression models with adjustment for age, parity, cervical cone biopsy, assisted reproduction and smoking. Associations between lifting and extremely (before 28 weeks), very (28–32 weeks) and moderately (33–37 weeks) preterm birth were analysed using Cox regression models. Results We found a dose–response relation between total daily burden lifted and preterm birth with an OR of 1.50 (95% CI 1.03 to 2.19) with loads over 1000 kg/day. No threshold value was found. The associations were strongest for extremely and very preterm birth with HRs (95% CIs) of 4.3 (1.4 to 13.8) and 1.7 (0.7 to 4.0), respectively. Lifting heavy loads (>20 kg) more than10 times/day was associated with preterm birth up to an OR of 2.03 (95% CI 1.14 to 3.62). Conclusion In a society with social welfare and a highly regulated working environment, occupational lifting was associated with an increased risk of preterm birth. INTRODUCTION According to antenatal guidelines in, for example, Denmark, Norway and the UK, pregnant women should not carry heavy loads. 1–3 These guidelines are formulated in broad terms and provide no information on a recommended maximum weight or maximum number of loads lifted per day during pregnancy. Previous Danish guidelines from 2002 suggested a maximum load of 12 kg and a maximum cumulative daily load of 1000 kg, with halved limits in the third trimester, 4 whereas recent guidelines from 2009 concentrate on lifting only as part of combined physical strain. 1 At the same time, daily physical exercise in pregnancy is encour- aged by health authorities in many countries. 156 Even though the association between occupa- tional lifting on the risk of preterm birth has been evaluated previously, 7–13 the picture is inconsistent. A review including 12 studies found a more than moderate adverse effect of lifting unlikely. 9 Relative risks (RRs) varied between 0.81 and 1.49 and were not statistically significant. If there is an effect of lifting heavy loads during pregnancy this may well differ according to the gestational age at birth. Recently, two studies were published from the Generation R birth cohort and the Southampton Women’s study, showing no association between heavy lifting and preterm birth. 14 15 The pathogenesis of preterm birth is not well understood. 16 It has been suggested that release of catecholamines into the blood stream in response to physical or psychological stress could lead to uterine irritability, cervical changes and preterm birth. 8 12 A weakened immune system due to stress Editor’s choice Scan to access more free content What this paper adds ▸ Antenatal counselling to pregnant women includes a general warning against lifting heavy loads during pregnancy, whereas recent guidelines from the Danish Working Environment Authority have omitted the previous explicit warning against lifting loads more than 12 kg each and a maximum cumulative daily load of 1000 kg. ▸ A review of the literature concludes that a more than moderate adverse effect of lifting during pregnancy is unlikely but the findings are inconsistent. ▸ Using data from the Danish National Birth Cohort we were able to estimate the risk of preterm birth according to occupational lifting of burdens among more than 62 000 women who were occupationally active during pregnancy and for whom self-reported data on work-related lifting were available. ▸ Heavy lifting was associated with an increased risk of preterm birth as a dose–response relation between cumulative daily load lifted and preterm birth risk was demonstrated. The associations were strongest for extremely and very preterm birth. ▸ As preterm birth is an adverse reproductive outcome with short-term and long-term consequences for the offspring, we think that heavy occupational lifting should be avoided during pregnancy. 782 Runge SB, et al. Occup Environ Med 2013;70:782–788. doi:10.1136/oemed-2012-101173 Workplace on June 3, 2020 by guest. Protected by copyright. http://oem.bmj.com/ Occup Environ Med: first published as 10.1136/oemed-2012-101173 on 9 July 2013. Downloaded from