OPK question

So I have been using OPKs for 3 days now to see if I will O for the first time since being off the pill. Last week I had a sonogram at the RE (Monday) and he said that my uterine lining was building and that I should ovulate in about 10 days (which would be Thursday). My first OPK, both lines were very clearly visible but the last two have been much lighter - does that mean I missed it? Do they get light then darker then lighter again? I am confused...

Re: OPK question

Yes, mine did and a couple of times the line seemed non-existant almost. They seem to vary in intensity throughout the testing period. If you want definitive proof however--use the Clearblue digital ones. There's no interpreting lines. It gives you a when you get your surge.