When looking at a due date calculator with a cycle length of 40days and a rough guess of 26th April as your last period then you would be about 8 weeks now. I struggled with mine due date too as MW refused to accept my 35 day cycle and would only look at the 28 day one.
but it measures your levels of HCG and the average ranges for your dates- actually cycle length is somewhat irrelevant - hence it giving you 2-3 weeks meaning 4-5 weeks pregnant as they just add on two weeks (as day 14 is averga ovulation day- i only seem to get pregnant when i ovulate "late"- day 24 or day 26 which means am glad I chart so I can date my pregnancies otherwise I would be in a right panic at scans not being as far along as I should!
Perhaps go along to your scan being convinced you are 5 weeks (and hence is okay to see a tiny blob without a heartbeat) and then you can be pleasantly suprised if you are dated further a long than you are. If you think about it doing it the other way round (dating 10 weeks and seeing a blob with no heartbeat) may cause a big panic. Good luck and let us know how the scan goes.
Oops a cocanut I'm suprised your MW did this- both my midwife and my doctor made up a new LMP for me to fit with my ovulation dates (Although for this pregnancy LMP was 29th Jan then last "bleed" was MMC at 10 weeks on 19th April and ovulated on 14th May so we made up an LMP of 1st May for the computers. )