First trimester employment, working conditions and preterm birth: A prospective population-based cohort study

Objectives: To explore the association between working conditions during first trimester and total preterm birth (PTB), and subtypes: Spontaneous PTB and iatrogenic PTB, additionally to explore the role of hypertension. Methods: Pregnant women from the Amsterdam Born Children and their Development study, filled out a questionnaire between January 2003 and March 2004, two weeks after first prenatal screening (singleton liveborn, n=7561). Working conditions were working hours/week, standing/walking hours/week, physical work load and job strain. Results: Prolonged standing/walking during first trimester was associated with an increased risk for total PTB (OR=1.5; 95% CI 1.0-2.3, after adjustments). Other working conditions were not related to total PTB. The separation into spontaneous and iatrogenic PTB revealed that standing/walking was associated with iatrogenic PTB only (OR=2.09; 95% CI 1.00-4.97). The highest risk was found for the combination of a long workweek with high physical work load (OR=3.42; 95% CI 1.04-8.21). Hypertension did not mediate these associations; however, stratified analysis revealed that high physical work load was only related to iatrogenic PTB when pregnancy-induced hypertension was present (OR=6.44; 95% CI 1.21-29.76). Conclusion: This study provides evidence that high physically demanding work is associated with an increased risk for iatrogenic PTB and not with spontaneous PTB. Pregnancy-induced hypertension may play a role: When present, high physical work load leads to a more severe outcome.