An endometrial thickness of 21mm after pregnancy is normal. During pregnancy after the fertilization of the egg, the thickness of the uterine lining may range from 8 to 40 mm, and most women’s uterine lining thickness ranges from 13 to 25 mm, so the thickness of the uterine lining after pregnancy is 21 mm is normal. Thickening of the uterine lining is a normal physiological change in order to allow the fertilized egg to settle smoothly, and also plays a supporting role in the early diagnosis of pregnancy, so it is a normal physiological process for the uterine lining to thicken to 21mm after pregnancy. The thickness of the uterine lining will continue to change during pregnancy. If the test results are abnormal, it is recommended to consult a doctor for treatment or therapy.