I couldn't remember either, so I tried to remembef when I had a period, then counted the days of a normal period and I was pretty close.. But yeah get an ultrasound and they will tell you. Congrats:)
Get an ultrasound in about three weeks, and they should be able to give you a reliable due date and tell you how many weeks gestational age you are. Keep in mind that when they say "x weeks" they are counting from the assumed first day of your last period, not the assumed day of conception.