Objective: Whether areas affected by Q fever during a large outbreak (2008-2010) had higher rates of adverse pregnancy outcomes than areas not affected by Q fever. Design: Nationwide registry-based ecological study. Setting: Pregnant women in areas affected and not affected by Q fever in the Netherlands, 2003-2004 and 2008-2010. Participants: Index group (N=58 737): pregnant women in 307 areas with more than two Q fever notifications. Reference group (N=310 635): pregnant women in 921 areas without Q fever notifications. As a baseline, pregnant women in index and reference areas in the years 2003-2004 were also included in the reference group to estimate the effect of Q fever in 2008-2010, and not the already existing differences before the outbreak. Main outcome measures: Preterm delivery, small for gestational age, perinatal mortality. Results: In 2008-2010, there was no association between residing in a Q fever-affected area and both preterm delivery (adjusted OR 1.01 (95% CI 0.94 to 1.08)), and perinatal mortality (adjusted OR 0.87 (95% CI 0.72 to 1.05)). In contrast, we found a weak significant association between residing in a Q fever-affected area in 2008-2010 and small for gestational age (adjusted OR 1.06 (95% CI 1.01 to 1.12)), with a population-attributable fraction of 0.70% (95% CI 0.07% to 1.34%). We observed no dose-response relation for this outcome with increasing Q fever notifications, and we did not find a stronger association for women who were in their first trimester of pregnancy during the months of high human Q fever incidence. Conclusions: This study found a weak association between residing in a Q fever-affected area and the pregnancy outcome small for gestational age. Early detection of infection would require mass screening of pregnant women; this does not seem to be justified considering these results, and the uncertainties about its efficacy and the adverse effects of antibiotic treatment.