Objectives Occupational exposures can increase the risk of preterm birth (PTB). We hypothesized that the risk varies significantly according to maternal work tasks and aimed to quantify relative risks and excess fractions of PTB related to occupation and industry during pregnancy.

Methods This nationwide study of 644 349 singleton pregnancies (2004–2018), classified maternal employment the year of conception into 38 occupational and 39 industrial groups. To approximate the lowest risk and excess fraction of PTB, ie, live birth before 37 completed gestational weeks, we constructed simulation-based reference groups that accounted for within-group random variation. Adjusted risk ratios (RR) and 99% confidence intervals (CI) of PTB were estimated relative to the simulation-based lowest risk reference and the population average.

Results Increased risks of PTB were observed for 22 occupational and 10 industrial groups, compared with the simulation-based lowest risks. The highest RR were found for assembly workers (1.42, 99% CI 1.12–1.79) and home care workers (1.35, 99% CI 1.24–1.47). For industries, the highest RR were found for agriculture, forestry and fishing (1.29, 99% CI 1.06–1.57) and manufacture of metals and machinery (1.22, 99% CI 1.07–1.40). Compared to the population average, risks were significantly higher for home care workers, nursing home workers, clerks (not elsewhere classified), shop assistants, and jobs with unstated job codes, and for the industrial groups of manufacture of metals and machinery and residential centers and home help.

Conclusion Several maternal occupational and industrial groups in Denmark had elevated risks of PTB. The findings are exploratory and further research is needed to identify the potential for prevention in the occupational setting.

Key terms Denmark; DOC*X Generation; excess fraction; maternal employment; maternal occupation; pregnancy; pregnant women; premature birth; preterm birth; register-based cohort study; registry study; relative risk

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