How does clearblue digital calculate dates

[MoonstoneGoddess]

Clearblue says 3+
So how exactly does it calculate this ?
I should be 6 weeks pregnant according to dates.
Clearblue calculates the actual date of conception. Do you add any weeks onto this ?
The way nhs count 4 weeks when period is due except for we know 2 of those your not actually pregnant but it's still counted.
If clearblue said 3+ yesterday and I'm 6 weeks does this mean my baby is measuring behind ?

It doesn't go any higher than 3+, don't worry. It's just the way they work, only goes up to 3+ on the wording, I think it says on the box something about it too irrc.