Table 1.

Key Variable Definitions.

Variable	Lesions Includeda		Definition
Acute inflammation	Maternal: acute subchorionitis, acute chorionitis, acute amnionitis, acute necrotizing chorioamnionitis Fetal: acute chorionic vasculitis, phlebitis, arteritis/panvasculitis, funisitis	High-grade	High stage maternal and/or fetal, based on Amsterdam Criteria
		Low-grade	Low-stage maternal and/or fetal, based on Amsterdam Criteria
		None	No acute inflammatory lesions
Chronic inflammation	Membranes/chorionic plate: chronic chorionitis, chronic amnionitis Basal plate: chronic deciduitis with plasma cells, chronic decidual perivasculitis Villi: villitis, intravillous plasma cells Intervillous space: chronic intervillositis Fetal: chronic eosin/t-cell vasculitis, chronic fetal inflammation- not specified	High-grade	≥2 compartments with chronic inflammation
		Low-grade	1 compartment with chronic inflammation
		None	0 compartments with chronic inflammation (no chronic inflammatory lesions)
Fetal vascular malperfusion	Thrombi or intramural fibrin deposition in chorionic vessel, velamentous vessel, stem villous vessel, and/or umbilical vessel; avascular villi/villous stromal vascular karyorrhexis	High-grade	High grade FVM or fetal thrombotic vasculopathy, defined as more than one focus of avascular villi (≥45 villi) or two or more occlusive or nonocclusive thrombi in the chorionic plate or stem villi, or multiple nonocclusive thrombi
		Low-grade	Any fetal vascular lesions, not indicated as high grade
		None	No fetal vascular lesions
Maternal vascular malperfusion	1 point: fibrinoid necrosis/acute atherosis, muscularization of basal plate arterioles, mural hypertrophy of membrane arterioles, basal decidual vascular thrombus, single infarct, increased syncytial knots, villous agglutination, increased perivillous fibrin deposition, distal villous hypoplasia, retroplacental blood/hematoma 2 points: multiple infarcts, retroplacental hematoma with hemosiderin or infarct, placental hypoplasiab	High-grade	Score of ≥4
		Low-grade	Score of 2–3
		None	Score of 0–1
Other significant pathology	Hemosiderosis, massive perivillous fibrin deposition, myonecrosis, amnion nodosum, delayed villous maturation, chorangiosis, isolated SGA placenta (placental hypoplasia)	Not graded	Presence of ≥1 lesion

^aSee Table S1 for specific lesion definitions.

^bPlacental hypoplasia only considered in the presence of ≥1 maternal vascular lesion (isolated placental hypoplasia not scored).