In England with my 1st I declared basically the day I found out
The second, after multiple losses I think I waited until I was closer to 9 as I didn't even tell my husband until 8 weeks (yay denial)
Delaying didn't delay any scans or appointments for me, but I think it depends on location and how busy the maternity unit is. It also didn't affect any emergency epu appointments I had as again, location wise, my epu then was self referral.
Third was after chemical pregnancy stage (5weeks?) so I could get scans easier with my community midwife but this one wasn't in England and epus aren't a thing here