Dating scan

An ultrasound has a margin of error of about two to three days. Especially early on, because measuring a millimeter differently can already make a big difference. But don't fixate on it too much. With me, it was off by five or six days once. Then she said that doesn't match (and in my case, it really couldn't have been, because we knew the exact date). But if you get pregnant naturally, your egg might have ovulated a bit earlier or later than you thought. So a one-day difference is totally possible anyway. They also said that they usually measure pregnancies as shorter rather than longer than you thought. When they're off by more days, they'll more likely subtract a week from your due date than add a week to it (except for people who have no idea when it probably happened).

First of all, congratulations on your pregnancy.
I was due on January 21st for the first ultrasound, January 23rd for the due date ultrasound, and in the end it was January 25th. So anything can happen, haha.

Thanks for your replies! A few days makes sense too. So if I understand you correctly, you can tell on a dating scan whether you're 9 or 11 weeks pregnant at the start?

At your due date scan, you get a due date. But that can still change, it's not always that exact date. Then you work towards that date, and you can give birth from 38 weeks up to 42 weeks.

The dating scan is at 12 weeks.
If they measure you as 8+6 during the early scan, they'll schedule the dating scan around 12 weeks. The fact that there might be a difference of maybe 2 days when there are exactly 3 weeks between the scans is just the margin of measurement. Every sonographer measures slightly differently, settings can vary, etc. And ultimately, very few babies are born exactly on the due date. Even if you know the exact conception date.

But if they measure 5+6 during the early scan (around 7/8 weeks), they'll only bring you in for the dating scan 6 weeks later.