Can postpartum soldiers attend promotion board? If yes, are they required to purchase new sets of ASUs due to pregnancy weight?

Yes and no. A post partum Soldier could attend the board, but they would normally be on post partum leave. You can't attend the board while you're on leave because you're not present for duty. Now, if the Soldier chooses to come off pregnancy leave and attend the board before the time the Army allocates to recover, then that is the Soldiers choice. What Soldiers have to wear to the board on any given month is at the discretion of the commander. I've seen promotion boards in ASUs, OCPs, and even body armor, Kevlar and weapon in an open field due to COVID.

I will let some of the Army folks on here answer whether you can attend the board but if you do don't be the one showing up in an ill fitting uniform. That's just drawing attention to yourself. If that means you need new tops or bottoms that is something for you to address.

Yes they can, and you can request to appear in OCP