Why do they add days in scans ?

Mm they don’t add days, they just count them.You are eg 5 weeks on a Monday then 5weeks+1,5weeks+2,5weeks+3, 5 weeks+4, 5weeks+5, 5weeks+6, then 6 on the following Monday. Depending when you do the scan eg Tuesday last week you would have been 5+1.It’s the gestational age of your pregnancy and it’s based on when you had your last period. This might change at your dating scan when will be based on the size of the baby that might of might not match (you ovulated before or after the usual 14 days)