Background <p>Prenatal depression shows diverse patterns in severity and progression. While air pollution has been linked to depression risk, its impact on the course of prenatal depression remains understudied. The study objectives were to (1) identify distinct trajectories of depressive symptoms across pregnancy, and (2) to evaluate whether preconception and early-pregnancy air pollution exposure impacts membership in trajectory groups.</p> Methods <p>This study followed 542 predominantly low-income Hispanic/Latina participants in the MADRES cohort. We assessed depressive symptoms using the Center for Epidemiologic Studies-Depression (CES-D) scale at 1st, 2nd, and 3rd trimester visits. Daily residential concentrations of PM_2.5, PM₁₀, NO₂, and O₃ were estimated using inverse-distance squared spatial interpolation from monitoring data. We used latent growth mixture modeling (LGMM) to identify trajectory classes and multinomial logistic regression to estimate adjusted relative risk ratios (RRRs) of membership in each class relative to the lowest group with averaged air pollution levels over preconception and early pregnancy.</p> Results <p>We identified four distinct trajectories: moderate-to-high symptoms (7.4%), consistently-moderate symptoms (14.6%), moderate-to-low symptoms (34.7%), and consistently-low symptoms (43.4%). Higher proportions of Black mothers and those not cohabitating were found in the two highest symptom trajectories. Participants with an interquartile range higher early-pregnancy NO₂ (10.28 ppb) had a 3.82-fold higher relative risk (95% CI: 1.50–9.73) of following the moderate-to-high versus consistently-low-symptom trajectory. Similar elevated risks were observed for PM_2.5 (RRR = 1.61; 95%CI: 1.08–2.40) and PM₁₀ (RRR = 1.84; 95% CI: 1.01–3.35).</p> Conclusion <p>Early-pregnancy exposure to ambient air pollution is associated with increased risk of moderate-to-high depressive symptoms across pregnancy.</p>