I wanted to interject here- I felt the first flutter at about 9 weeks, so it can happen even earlier than 13 weeks. but 20 weeks is not abnormal either. many women say they felt it earlier on the second baby, maybe due to knowing what they were feeling? I should also note that this is my first, but I knew it wasn't my intestines doing that wiggling around at 9 weeks
generally, you measure within a few centimeters of your week. 30 cms for 30 weeks, etc. lots of things can affect this, like amount of amniotic fluid, mother's size before baby, how the baby is deciding to lie on a particular day.
O is a good measure for predicting conception, but when you actually had sex is the best predictor of all
I became pregnant when I should have been starting my period (though I confess I wasn't keeping track either really, I just happened to remember the last flow b/c it happened right before an important trip I was taking) I know I ovulated about 2 week prior, but I didn't have any sex then, so I had to ovulate again, and no period. so what am I saying? babies happen when they want to happen, O date or no.
if an ultrasound is matching up to your tape measurements then I think you can be pretty sure about this EDD.
then again, you could still gestate 38 weeks, or 42.... not all babies cook the same.
I realize this might not be too helpful, sorry. I guess I think that if you intuit that everything is on track and growing well, then it is.