I am a little over 9 weeks pregnant, and I tested with an OPK first. It wasn't as dark as the control line. After that, I did an HPT, and it was instantly positive. For me, when I wasn't pregnant, the second line on OPKs didn't show up, so it was weird for it to come up--and that is why I finally tested with an HPT. I have tested with OPKs that I have left over randomly so far in my pregnancy just to see, and they definitely got darker as I went. Now they are to the point that they can't get any darker! Point being that it is a good (but not definitative) sign that you are showing a line on the OPK. Testing with an HPT is definitely the only sure way to know.