Hazard ratios for preterm birth in relation to total load lifted per day among primi- and multigravid women.

* Adjusted for maternal age at conception.** Adjusted for maternal age at conception, conic section, assisted reproduction, pre-pregnancy body mass index, and in pregnancy: smoking and alcohol consumption.***In addition for nulliparity and previous spontaneous abortion among multigravid women.¤Due to missing information on primi-/multigravid status, 37 women were excluded from these analyses.