Shift work and subfecundity: a causal link or an artefact?

Aims: The Danish National Birth Cohort ( DNBC) was used to examine whether shift work is associated with reduced fecundity as estimated by time to pregnancy ( TTP). Methods: From 1 March 1998 to 1 May 2000, 39 913 pregnant women were enrolled in the DNBC. Data on job characteristics and TTP ( 0 - 2, 3 - 5, 6 - 12, and > 12 months) were used for 17 531 daytime workers and 3907 shift workers who had planned the pregnancy. Fecundity odds ratios ( ORs) were calculated with 95% confidence intervals using the discrete time survival analysis techniques performed by logistic regression. An OR above 1 expresses a shorter TTP and then a higher fecundity. Potential confounders, such as age at conception, gravidity, prepregnant body mass index, smoking, and alcohol consumption, as well as occupational characteristics, were also included in the model. Results: Fixed evening workers and fixed night workers had a longer TTP. Compared with daytime workers, the adjusted ORs were 0.80 ( 95% CI 0.70 to 0.92) for fixed evening workers, 0.80 ( 95% CI 0.63 to 1.00) for fixed night workers, 0.99 ( 95% CI 0.91 to 1.07) for rotating shift ( without night) workers, and 1.05 ( 95% CI 0.97 to 1.14) for rotating shift ( with night) workers. When analysis was restricted to nulliparous women, the estimates remained unchanged. The proportions of unplanned pregnancies and contraceptive failures were higher among fixed evening and fixed night workers. Conclusions: There was no unequivocal evidence of a causal association between shift work and subfecundity. The slightly reduced fecundity among fixed evening workers and fixed night workers may be mediated by pregnancy planning bias or differential options for sexual contacts.