Inorganic arsenic exposure has been linked to the development of several health conditions, including adverse birth outcomes, and around 150 million of people worldwide are exposed to levels above the WHO suggested limit of 10 μg/L. A recent risk assessment in pregnant women of Tacna, of this same population performed by our group, found that 70.25% were exposed to arsenic concentrations in drinking water ≥ 25 μg/L. The present study aimed to evaluate the relationship between prenatal total urinary arsenic (U-tAs) and inorganic arsenic (U-iAs) with adverse birth outcomes. A total of 147 pregnant women from the province of Tacna, Peru, during February–March, 2019, were evaluated for U-tAs and U-iAs exposure during their second trimester of pregnancy, while the birth records of their children were collected from the local hospital. The geometric mean U-tAs was 43.97 ± 25.88 μg/L (P50 22.30, range 5.99–181.94 μg/L) and U-iAs was 5.27 ± 2.91 μg/L. Controlling for maternal age, pre-pregnancy BMI, parity, mother’s education, and newborn sex, no relationship was observed between tertile of U-tAs and the birth outcomes considered, although we found an apparent but statistically non-significant dose–response relationship for small for gestational age 2.38% (95% CI 0.003, 0.16), versus 7.32% (95% CI 0.02, 0.21%), and versus 8.57% (95% CI 0.03, 0.25%). This finding requires further evaluation considering other factors such as metabolic arsenic species, additional maternal covariates, and ethnicity.