cherry83, baby center usually calculates based on LMP, ie, your last menses.. and it's based on normal ovulation which is 14 days after menses.. which means baby will be 4 weeks exactly a month from your menses..
however, if your ovulation is late by 1 or 2 weeks, the you baby will only be 4 weeks at 6th weeks after your menses...
it happened to me during my #1..and the first gynae that i went to (he goes by the books) actually told us that it should be miscarriage coz cannot see during initial scan..
i guess u wait for your blood test results to confirm.. gotta bear with it for another week i guess..