We examined whether exercise during pregnancy is associated with chromosomally normal spontaneous abortion. We hypothesized that associations would be confined to chromosomally normal losses since only these can be influenced appreciably by experiences during pregnancy. The study population comprises women with chromosomally normal (N = 173) and aberrant (N = 173) losses. Women who exercised during pregnancy had a lower risk of a chromosomally normal spontaneous abortion (OR 0.6; 95% CI 0.3-0.9). Other types of physical activity had little effect.