This is possible but is obviouly isn't the norm by any means - sometimes people will have a couple days of flow of something called "serous fluid" which is the clear component of blood, before AF begins. It may still be classed by your body as AF flow though even though the red portion of the lining doesn't start to come off for a couple days after that. the lining can start to regrow even as the old lining comes off, so if a person had a couple days of fluid, then bled for a few days after that they may have been misled into thinking they were earlier in the cycle than they really were and could get pg even on what appeared to be CD 5-6 range.