NCIthesaurus neonatal anemia twin-to-twin transfusion syndrome SNOMEDCT_US_2025_09_01:13404009 MESH:D005330 ICD10CM:O43.02 Fetal blood loss from fetal hemorrhage into co-twin UMLS_CUI:C2909036 NCI:C113824 disease_ontology DOID:13576 A neonatal anemia that is characterized by an increased number of arteriovenous anastomoses deep in the placenta. These capillary connections occur in the cotyledon portion of the placenta. Unidirectional flow can occur in these arteriovenous anastomoses and result in shunting of blood towards one twin and away from the other when the arteriovenous anastomoses are unbalanced. Fetal hemorrhage into co-twin Twin to twin transfusion Twin-to-twin blood transfer