How long are your cycles? the length of your cycle can make a difference to EDD - I have a 34 day cycle so the 28 day EDD estimators don't work for me and give me a due date a week earlier.

Also, you usually concieve two weeks before your AF is due -some of the calculators give you a date when conception would have happened too. Have you tried the BellyBelly calculator? it allows you to change the length of your cycle and your lutal phase (the time between Ovulation and AF)