Dating pregnancy irregular periods
Just because your cycles are irregular, I can't imagine that they wouldn't use the first day of your LMP to date you at first, but maybe someone else can give some insight.
If your cycle is very irregular they really can't use your LMP because it will be wildly inaccurate.
For example, when I was pregnant with my first I didn't ovulate until around cycle day 70.
They use that first dating ultrasound to determine your due date etc.
To answer your other question, when they measure the fetus, it includes that phantom two weeks that is factored in before you're actually pregnant.
I knew when I ovulated, I knew when I started testing positive...
I knew I was 4 weeks-ish, not 12 like my actual LMP would have suggested!