So my first ever pregnancy I had a missed miscarriage at 11 weeks the baby was only measuring at 8.

Now fast forward 5 and a bit months and I'm pregnant again finally, but because of prior miscarriage my doctor wanted me to go in at when I should be 8 weeks according to when I last had my period but now she wants me to go in at 7 weeks, I'm just wondering what you see at these times? if the dating is accurate ? And if it isn't what would you see if it's say at 7 weeks it's actually 5 and so on?