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Background Occupational physical activity (OPA) during pregnancy has been linked to adverse pregnancy outcomes, but crude exposure assessment remains an issue in causal inference. We aimed to develop a quantitative trimester-specific job exposure matrix (JEM) for standing, walking, and forward bending, among pregnant workers.Methods Accelerometer measurements from 403 female workers within 109 DISCO-08 job codes were obtained between January 2023 and June 2024 in Denmark. Full workdays were measured during two weeks in pregnant workers and one week in non-pregnant workers. To estimate exposure levels of occupational standing, walking and forward bending for all 1171 DISCO-08 codes, we included expert ratings, age and trimester as fixed effects, and job codes and workers as random effects in linear mixed-effects models.Results The between-job variances relative to total variances were 56% for standing, 51% for walking and 45% for forward bending. The fixed effects age and trimester reduced standing time with 0.02 hours/year, and 0.38 hours during 3rd trimester, respectively whereas no differences were observed for walking or forward bending. Including expert ratings in the final models reduced between-job variances with 50% for standing and forward bending, and 17% for walking. Based on the trimester-specific JEM for standing, bakers had the highest exposure (range 5.41-5.03 hours). For walking and forward bending, the highest exposed jobs were waiters (1.76 hours) and livestock/dairy producers (1.24 hours), respectively.Conclusion The JEM enhances independent objective exposure assessment in epidemiological studies of OPA and pregnancy outcomes and may advance guidelines and potentially prevent adverse pregnancy outcomes.