Table 1. Different classification systems of endometrial hyperplasia.

Classifying year	Classifying type				Source
1961	Benign hyperplasia	Atypical hyperplasia type I	Atypical hyperplasia type II	Atypical hyperplasia type III	[25]
1963	Mild adenomatous hyperplasia	Moderate adenomatous hyperplasia		Marked adenomatous hyperplasia	[26]
1966	Cystic hyperplasia	Adenomatous hyperplasia	Anaplasia	Carcinoma in situ	[27]
1972	Cystic hyperplasia	Adenomatous hyperplasia	Atypical hyperplasia	Carcinoma in situ	[28]
1978	Cystic hyperplasia	Adenomatous hyperplasia		Atypical hyperplasia	[29]
1979	Hyperplasia without atypia	Hyperplasia with mild atypia	Hyperplasia with mild atypia	Hyperplasia with severe atypia	[30]
1985	Simple, nonatypical	Complex, nonatypical	Simple atypical	Complex atypical	[13]
WHO (1994)	Simple hyperplasia	Complex hyperplasia	Simple hyperplasia with atypia	Complex hyperplasia with atypia	[31]
WHO (2003) (revised)	Proliferative endometrium ● Tubular and regularly spaced gland ● Glands are lined with pseudostratified nuclei ● Abundant stroma ● Mitotic figures are easily found both in glands and stroma	Simple hyperplasia ● Irregular shape and size glands ● Cystic appearance ● Abundant stroma, ● Nuclear pseudostratified glands but no nuclear atypia ● No back to back crowding	Complex hyperplasia ● Closely packed glands ● Stroma is relatively sparse ● Gland to stroma ratio is more than 2:1 ● Nuclei are uniform, oval and pseudostratified ● Nucleoli are indistinct	Complex atypical hyperplasia ● Tightly packed glands ● Very little intervening stroma ● Larger and vascular nuclei with chromatin clumped along the nuclear membrane ● Prominent nucleoli	[15]
EIN	Benign or endometrial hyperplasia	EIN		Carcinoma	[23,24]

Adapted from Trimble et al. [32], with permission from Wolters Kluwer Health, Inc.

EIN, endometrial intraepithelial neoplasia.