It is diagnosed by a microscopic examination of the placenta.
Commonly used criteria from Altshuler[2][3] are: "a minimum of 10 villi, each with 10 or more vascular channels, in 10 or more areas of 3 or more random, non-infarcted placental areas when using a ×10 ocular." The Altshuler criteria are not theoretically rigorous, as they do not define the area. Normal villi have up to five vascular channels.[3]
^Altshuler, G. (Jan 1984). "Chorangiosis. An important placental sign of neonatal morbidity and mortality". Arch Pathol Lab Med. 108 (1): 71–4. PMID6546343.