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Endometrial thickness varies from 6 to 12 mm and depends on the phase of the reproductive cycle.Just before the periods the thickness of endometrium increases and can go up to 12mm but after the periods it is usually thin as the endometrial linins is shed.Heavy bleeding during periods can be indicative of some underlying disorder like Endometritis,Endometrial Hyperplasia etc.Prominent endometrium after the periods can be indicative of some abnormality and curettage(D&C)helps in finding the exact cause and also treating it.
Kindly follow as your treating doctor is guiding you.
Take Care!