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.