BACKGROUND: Low- and middle-income countries bear a disproportionate burden of preterm birth (PTB) and low infant birth weight (LBW) complications where affective and anxiety disorders are more common in the antepartum period than in industrialized countries. OBJECTIVE: To evaluate the extent to which early pregnancy antepartum depression, generalized anxiety disorder, and posttraumatic stress disorder (PTSD) are associated with infant birth weight and gestational age at delivery among a cohort of pregnant women in Peru. METHODS: Our prospective cohort study consisted of 4408 pregnant women. Antepartum depression, generalized anxiety, and PTSD were assessed in early pregnancy using the Patient Health Questionnaire-9, Generalized Anxiety Disorder Scale-7 and PTSD Checklist - Civilian Version, respectively. Pregnancy outcome data were obtained from medical records. Multivariable linear and logistic regression procedures were used to estimate adjusted measures of association (β coefficients and odds ratios) and 95% confidence intervals (CI). RESULTS: After adjusting for confounders, women with antepartum generalized anxiety (32.6% prevalence) had higher odds of LBW (adjusted odds ratio (OR)=1.47; 95%CI: 1.10-1.95) and were more likely to deliver small for gestational age (OR = 1.39; 95%CI: 1.01-1.92) infants compared to those without anxiety. Compared to those without PTSD, women with PTSD (34.5%) had higher odds of delivering preterm (OR = 1.28; 95%CI: 1.00-1.65) yet PTSD was not associated with LBW nor gestational age at delivery. Women with antepartum depression (26.2%) were at no increased risk of delivering a preterm, low-birth-weight or small-for-gestational-age infant. LIMITATIONS: Our ability to make casual inferences from this observational study is limited; however, these findings are consistent with prior studies. CONCLUSION: Generalized anxiety disorder during pregnancy appeared to increase odds of delivering a low-birth-weight or small-for-gestational-age infant, while PTSD was associated with increased odds of delivering preterm. Our findings, and those of others, suggest antenatal care should be tailored to screen for and provide additional mental health services to patients.