© 2018, © 2018 Informa UK Limited, trading as Taylor & Francis Group. Antepartum suicidal behaviors are a leading cause of maternal injury and death. Previous research has not investigated associations between antepartum suicidal ideation and perinatal complications. Our study objective was to evaluate the relationship of antepartum suicidal ideation with low infant birthweight, small for gestational age, and preterm birth. A cohort study was conducted among 1,108 women receiving prenatal care in Peru. Suicidal ideation was measured using the Patient Health Questionnaire-9 during pregnancy. Birth outcomes were extracted from medical records. Linear regressions and multivariable logistic regressions were used to estimate were used to investigate associations between suicidal ideation and pregnancy outcomes. The prevalence of suicidal ideation was 8.7%, preterm delivery was 5.7%, low birthweight was 4.4%, and small for gestational age was 3.4%. In an adjusted model, infant birthweight was 94.2 grams lower for mothers with antepartum suicidal ideation (95% CI: −183.0, −5.5, p = 0.037) compared with those without suicidal ideation. After adjusting for confounders including depression, participants with suicidal ideation had a nearly four-fold increased odds of delivering a small for gestational age infant (OR: 3.73; 95% CI: 1.59–8.74). These findings suggest suicidal ideation during pregnancy is associated with adverse perinatal outcomes, especially low infant birthweight.