Hey, just wanted to congratulate you, sj on your twins!! How wonderful.
Hmmm, I really don't know. I'd be more inclined to go by my LMP dates if I were you, unless your cycle is irregular? 1-4 days after your period to O seems a bit soon to me, but I'm no expert.
My dr told me (when I went for my scan at 13 weeks) that they can get the dates pretty accurate within about a week. So it sounds to me like if you are measuring about a week bigger, your dates are still pretty right, KWIM?
Try not to stress! Enjoy your pregnancy! Join in on the twins or more forums if you like, and join up with your belly buddies group - they are great for sharing your pregnancy experiences with one another All the best for a wonderful pregnancy.