Objective To establish exposure estimates based on prospective interviews in a cohort of pregnant women using a job and industry group-based approach, and to evaluate whether non-occupational factors should be considered when a group-based strategy is chosen based on self-report.

Methods Within the Danish National Birth Cohort, a job exposure matrix (JEM) based on information from participants still pregnant at date of interview had been constructed. Job titles were coded using the Danish version of the International Standard Classification of Occupations (DISCO-88) and the Danish Industrial Classification of All Economic Activities of employment industry was obtained from Statistics Denmark. We calculated the mean number of kilograms lifted per day within groups defined firstly by identical four digits DISCO-88 codes and employment industry, secondly by identical four, three or two digits DISCO-88 codes, depending on a compulsory group size of ≥10 participants. Level of self reported occupational lifting in relation to JEM-estimate and non-occupational factors including gestational age at interview, mother’s age, parity, smoking and body mass index (bmi) was analysed with restriction to participants with identical job and industry codes. We used multivariate regression with bootstrap to obtain regression coefficients and 95% confidence intervals.

Results A total of 41,405 women were included in the analyses. Gestational age >97 days at interview and increasing mother’s age were associated with reporting of lower levels of occupational lifting, whereas bmi ≥30 kg/m², smoking while pregnant, and being a mother was associated with reporting of higher levels. Differences ranged from -15 kg to 35 kg.

Conclusion We constructed a JEM based on self-report to obtain group-based exposure estimates, striving to minimise information bias and attenuation in exposure response analyses. Non-occupational factors influence reporting of occupational lifting among pregnant women. This should be considered when a group-based strategy is used for exposure assessment based on self-report.