When I first went into the doctors, I was about 2 months. At that point the told me at my three month visit that it will be a total check up (including a pap test).
I don't think it could induce labour, specailly only at 6 months.
I think they do a pap test to see if anything is wrong, then if they find out early they can help the baby be okay.
Different doctors probably just do the test at different times, but I couldn't see it effecting anything or they wouldn't do it.