BACKGROUND: Infants born small for gestational age because of pathologic placenta-mediated fetal growth restriction can be difficult to distinguish from those who are constitutionally small. Additionally, even among fetal growth-restricted pregnancies with evident placental disease, considerable heterogeneity in clinical outcomes and long-term consequences has been observed. Gene expression studies of fetal growth-restricted placentas also have limited consistency in their findings, which is likely due to the presence of different molecular subtypes of disease. In our previous study on preeclampsia, another heterogeneous placenta-centric disorder of pregnancy, we found that, by clustering placentas based only on their gene expression profiles, multiple subtypes of preeclampsia, including several with co-occurring suspected fetal growth restriction, could be identified. OBJECTIVE: The purpose of this study was to discover placental subtypes of normotensive small-for-gestational-age pregnancies with suspected fetal growth restriction through the use of unsupervised clustering of placental gene expression data and to investigate their relationships with hypertensive suspected fetal growth-restricted placental subtypes. STUDY DESIGN: A new dataset of 20 placentas from normotensive small-for-gestational-age pregnancies (birthweight <10th percentile for gestational age and sex) with suspected fetal growth restriction (ultrasound features of placental insufficiency) underwent genome-wide messenger RNA expression assessment and blinded detailed histopathologic evaluation. These samples were then combined with a subset of samples from our previously published preeclampsia cohort (n=77) to form an aggregate fetal growth-focused cohort (n=97) of placentas from normotensive small-for-gestational-age, hypertensive (preeclampsia and chronic hypertensive) small-for-gestational-age, and normotensive average-for-gestational-age pregnancies. Gene expression data were subjected to unsupervised clustering, and clinical and histopathologic features were correlated to the identified sample clusters. RESULTS: Clustering of the aggregate dataset revealed 3 transcriptional subtypes of placentas from normotensive small-for-gestational-age/suspected fetal growth-restricted pregnancies, with differential enrichment of clinical and histopathologic findings. The first subtype exhibited either no placental disease or mild maternal vascular malperfusion lesions, and, co-clustered with the healthy average-for-gestational-age control subjects; the second subtype showed more severe evidence of hypoxic damage and lesions of maternal vascular malperfusion, and the third subtype demonstrated an immune/inflammatory response and histologic features of a maternal-fetal interface disturbance. Furthermore, all 3 of these normotensive small-for-gestational-age subtypes co-clustered with a group of placentas from hypertensive small-for-gestational-age pregnancies with more severe clinical outcomes, but very comparable transcriptional and histologic placental profiles. CONCLUSION: Overall, this study provides evidence for at least 2 pathologic placental causes of normotensive small-for-gestational-age, likely representing true fetal growth restriction. These subtypes also show considerable similarity in gene expression and histopathology to our previously identified "canonical" and "immunologic" preeclampsia placental subtypes. Furthermore, we discovered a subtype of normotensive small-for-gestational-age (with suspected fetal growth restriction) with minimal placental disease that may represent both constitutionally small infants and mild fetal growth restriction, although these cannot be distinguished with the currently available data. Future work that focuses on the identification of etiology-driven biomarkers and therapeutic interventions for each subtype of fetal growth restriction is warranted.