Dear riversedge: Thanks for awaiting a reply. As you have correctly stated, any tiny fragment of uterine lining which escaped destruction from an endometrial ablation (EA) can begin to regrow and "re-populate" the inside of the uterine cavity. This is illustrated by your very slow return of some menstrual type bleeding.
Simply stated, if the uterine lining is like grass or lawn, estrogen is like the fertilizer (causes a thickening of the lining), and progesterone is like the lawnmower (keeps the lining thin by three different mechanisms). This is why DepoProvera (high dose synthetic progesterone) brings about a thin lining, and why birth control pills (relatively progesterone dominant) bring about shorter, lighter periods. At your stage of life (entering perimenopause) estrogen levels can actually surge higher (more stimuation to the lining). Ovulations become less frequent, and if they do occur, less progesterone can be produced. The most common reasons for a woman to not be ovulating (no lawn mower activity so thicker lawn) are being heavy set or being early perimenopausal.
The ultrasound image measures the thickness of the lining ("endometrial stripe"); with a "too thick" lining (endometrial hypertrophy) it is usually more than 12-15 mm. I had a patient once with a stripe of 30mm! The endometrial biopsy takes a sample of lining tissue and the pathologist can then make a very accurate diagnosis base upon the cells seen under the microscope. The treatment for simple hyperplasia/hypertrophy which does not contain abnormal cells is synthetic progesterone pills or shots. This shrinks down the too thick lining.
Yet another cause of an apparently "too thick" lining would be multiple polyps of the uterine lining. Generally this appears as a more irregular kind of surface.
Usually a thickened lining does not prompt back or pelvic pain. Hopefully your GYN appointment will be able to give you the most "for sure" answer as they will have access to all your records..
Yours,
Jane