If you look online, it says it goes from conception. It starts at week 1 and at week one your not even pregnant when you go by lmp. But, like others have said, there is such a huge range it really won't tell you how far along you are, altho I guess it could give an estimate. With my daughter, mine was 18,762 at 4w6d, but had I gone by the charts I would have thought I was further along (which I wasn't bc I had a scan the following week and it confirmed the date) gl