An additional tenth of one part-per-million daily average exposure to ozone is associated with a 33 g decrease in birthweight. This decrease in birthweight is about the same size as the association of gestational diabetes (32 g), larger than the association of chronic hypertension (22 g), and about 40% the size of the effect of smoking during pregnancy on birthweight (84 g). Given the magnitudes of the associations with atmospheric ozone and adverse birth outcomes, reducing atmospheric ozone should be a public health priority. Inclusion of controls for housing cost-burden and poor-quality housing reduces the magnitude of the association with mothers who identify as Black, suggesting the importance of these structural factors in understanding adverse birth outcomes by race.