Objective A number of studies examined the effects of prenatal stress on birth outcomes with diverging and inconclusive results. We aimed to examine if working with high job strain during pregnancy measured in week 16 was associated with risk of giving birth to a child born preterm or small/large for gestational age (SGA/LGA), and second, if social support affected any associations.

Design Study population was 48 890 pregnancies from the Danish National Birth Cohort. Multinomial logistic regression estimated ORs. Covariates included: maternal age, BMI, parity, exercise, smoking, alcohol and coffee consumption, manual work, serious maternal disease, parental height and gestational age at interview. In accordance with Good Epidemiological Practice, a protocol outlined the study design before analyses were initiated.

Results High job strain was associated with significantly lower odds of being born LGA (OR=0.81, CI 0.70 to 0.92) when compared to low-strain jobs, but no associations between high strain and preterm birth or SGA were found. Stratification by social support showed a non-statistically significant tendency of higher odds of preterm birth when exposed to high strain and low social support.

Conclusions In spite of the high statistical precision, we did not find any clear association between job strain and preterm birth nor between job strain and SGA.