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Adverse pregnancy outcomes are significant public health issues in developing countries. This study aims to evaluate the trimester-specific impact of COVID-19 infection on maternal and neonatal outcomes in a cohort of unvaccinated Iranian women. A multi-center cross-sectional study was conducted between March 21, 2020, and March 21, 2021, involving 217 unvaccinated pregnant women with RT-PCR-confirmed SARS-CoV-2 infection who delivered in hospitals across three counties in Northwest Iran. Participants were stratified by the trimester of COVID-19 diagnosis: first (n = 20), second (n = 87), and third (n = 110). Data on demographics, as well as maternal, obstetric, and neonatal outcomes, were extracted from the National Health System. Statistical analyses included ANOVA, Chi-square, Fisher's exact and Kruskal-Wallis tests, with post-hoc Bonferroni corrections. A significant association was found between the trimester of infection and the rate of PTB (p = 0.028). Women infected in the third trimester had a substantially higher PTB rate (26.4%) compared to those infected in the second (11.5%) and first (15.0%) trimesters. Neonatal outcomes also varied significantly; APGAR scores at 1 and 5 minutes were lowest in the third-trimester infection group (8.16 ± 0.78 and 8.39 ± 0.80, respectively) compared to the first (9.08 ± 0.88 and 9.03 ± 0.90) and second (8.45 ± 0.71 and 8.79 ± 0.83) trimesters (p < 0.001 for both). Fever was significantly more prevalent in the third trimester (33.6%) than in the first (10.0%, p = 0.013). No vertical transmission or neonatal deaths were recorded. This study demonstrates a clear trimester-specific risk gradient for SARS-CoV-2 infection in unvaccinated pregnant women. Infection during the third trimester was associated with a significantly increased odds of PTB and lower neonatal APGAR scores(1,5). These findings underscore the critical vulnerability of late pregnancy to COVID-19 complications and highlight the importance of trimester-specific vigilance, enhanced antenatal surveillance, and robust vaccination advocacy for pregnant women.