INTRODUCTION: The purpose of this study was to compare microscopic placental characteristics between preterm twins and singletons, and between preterm monochorionic and dichorionic twins, in order to explore the effect of placental pathology on adverse neonatal outcomes. METHODS: This study included 566 neonates born ≤32 weeks and/or ≤1500 g, of whom 429 were singletons and 137 were twins (38 monochorionic and 99 dichorionic). Clinical data was retrospectively collected, and placentas were prospectively examined for maternal vascular malperfusion, fetal vascular malperfusion and placental inflammation (acute and chronic). RESULTS: Singletons had increased rates of maternal vascular malperfusion, fetal hypoxia, funisitis (in umbilical cord and chorial plate), chronic deciduitis, and villitis of unknown etiology compared to twins. Delayed villous maturation and ischemia were more frequently present in monochorionic placentas than in dichorionic. Singletons had a significant lower birthweight and were more often small for gestational age than twins. Multivariate linear regression analysis adjusting for singleton pregnancy, gestational hypertension and placental abnormalities showed that gestational hypertension (β = -114.8), infarct (β = -130.1), decidual necrosis (β = -115.4), fetal hypoxia (β = -59.3) and chronic deciduitis (β = -118.8) were independently associated with lower birthweight. Multivariate regression analysis revealed five independent risk factors of small for gestational age: gestational hypertension (OR 4.4), infarct (OR 3.7), decidual necrosis (OR 2.7), fetal hypoxia (OR 1.9) and villitis (OR 5.2). DISCUSSION: Singleton pregnancies vary in histological placental abnormality rates from twin pregnancies. This study demonstrated that differences in birthweight and small for gestational age rates between preterm twins and singletons can be attributed to gestational hypertension and histological placental abnormalities.