Dear Mish: We appreciate your reading the questions of others to see if your situation was similar. Now, because your regimen is different, YOUR specific information may be useful to someone else.
As you have read, the consistent exposure of the lining of the uterus to progesterone (either natural from the ovary of synthetic) causes the lining to decrease in thickness. 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. It is also why women who miss ovulations (no progesterone produced) are at a greater risk for too thick lining, heavy/prolonged flows and if untreated, endometrial cancer.
So in your case Mish, taking the Provera pills every day except for the last four days of the month gives you a dose and effect similar to getting the DepoProvera injection. After four days without the Provera pills your progesterone blood level drops enough to trigger any lining to be shed--if needed. That you don't have a withdrawal bleed suggests that your lining is being kept thin enough that there is not much that needs to be shed.
In terms of the TTC issues, Provera will not prompt an established pregnancy to be lost. While on DepoProvera shots for birth control one is exposed to the equivalent of Provera 20-25 mg daily for 90 days. That dose does suppress ovulations so conception should not occur. I do not know whether your dose of Provera 10 mg x 24 days will suppress ovulations as well. With your known history of PCOS, ovulations are classically fewer than monthly.
Mish, your GYN can give you the most "for sure" answer. You are definitely asking all the right questions. In my OPINION, it sounds like your GYN is first attempting to get your lining to a healthy level so you don't have to be hospitalized. In terms of TTC, you might be a candidate for ovulation induction medications.
Yours,
Jane