An endometrial lining of 21 millimeters is relatively thick and needs to be judged on a case-by-case basis as a possible sign of abnormality, while pregnancy needs to be ruled out. The thickness of the endometrium varies at different stages of the menstrual cycle. Generally, the endometrium is thinnest during the menstrual period, with a thickness of about 0.5mm; during the proliferative period, the thickness of the endometrium grows from 0.5mm to 3-5mm; during the secretory period, the endometrium is the thickest, and can reach about 10mm. Therefore, an endometrial thickness of 21mm belongs to endometrial thickening, which may be an abnormal manifestation caused by abnormal ovulation function or endometrial cancer and other diseases. Meanwhile, it should be noted that if the endometrial thickness reaches 21mm in pregnant women, it may be normal. Therefore, it is recommended to consult a doctor in time to improve the diagnostic scraping and other related examinations to clarify the condition and standardize the diagnosis and treatment.