can anybody help?

I'm booked in for an early scan on Friday, It will be 6 weeks and 2 days since the start of my last period. Why do they date pregnancies from your last period tho because surely my pregnancy will only measure about 4 weeks or so from my ovulation? I'm only going to see if this pregnancy is a twin pregnancy again but I'm worried they won't pick up a heartbeat yet? Has anyone else ever had a very early scan?