I got pregnant pretty much straight away after coming off the pill (i think a week later or something) - didn't think it was going to happen that quickly so i got a big suprise!

I actually O a week later, because all my preg things said LMP + 1 week, only after my obs did a dating scan as i was trying to convince him i couldn't be as pregnant as he thought i was (DH was away then )

I would ask for a scan around 6 weeks, although my EDD from that was only 1 day of the EDD from my 19 week scan, but apparently it is harder to tell later on.

HTH