Thickening of the uterine lining can occur before menstruation or after pregnancy, and you can’t tell if you’re pregnant or menstruating simply from the fact that your uterine lining is 13mm thick. The lining of the uterus will thicken from thin to thick after pregnancy and will change with the hormonal changes in the body during pregnancy. A urine pregnancy test or blood test for chorionic gonadotropin can be done to determine whether you are pregnant or not, and you cannot determine pregnancy from the thickness of the lining of the uterus. Under normal circumstances, endometrial thickness of more than 10mm is in the late stage of secretion, suggesting that menstruation is imminent. Endometrial lining 13mm may also be endometrial hyperplasia is too thick, need to go to the hospital for examination, check for any abnormal changes, under the guidance of the doctor for treatment.