Celynlee,
when they do the initial sonogram, they will be able to tell how many weeks pregnant you are.
my guess that they count from the 1st day and not last day of menses is becos the minute the menses comes, it means u are not pregnant( usually lah), so the following mth, if it's supposed to be here, but not, that means egg has been "fertilised" liao, so use the LMP lor.. hahah.. sounds confusing hor? anyway, that's my opinion lah. but then that really only applies if ur period is the accurate kind.. which is why doc may tell u that u are actually XX weeks instead of YY weeks. ( based on sonogram)
btw, how are u feeling now?