my friend waited 6 weeks to get clearance from her OB, then her first ride back she fractured her knee! (can't remember bones involved etc).
In her case it was suggested that her calcium was depleted after giving birth, and another point I heard was that for about 3 months after giving birth, all ligaments etc in the body are extra loose, which may have allowed extra movement in the joint and added to the injury.
my suggestion would be to wait for drs clearance, take some calcium supplements, and gradually build up to more high impact riding (ie don't try to start jumping right away!).
I think my friend was unlucky, but it was such a random thing to happen, and it wasn't a bad fall at all. just be careful