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.