If you tell your obgyn you are not sure, they can take measurements during your ultrasound. Don’t hesitate, because as your time passes, the variance becomes greater. I had my first US at 12 weeks 1 day and they say at this point it is accurate plus or minus 7 days. If you get in around week 8, they might be able to guess within 3 days according to what I have read.

And congratulations!!!