Hai! Yes, exactly, line akan gradually terang as you get closer to your LH surge, so it's very normal yang CD12 samar and CD13 lebih terang already, means your body is gearing up! Biasanya the darkest line tu peak fertility, so monitor je sampai line tu sama darkness or darker than the control line (that's the positive), then ovulation usually happens within 12-24 hours after that surge. A few of my mum friends doing OPK timing juga noticed the progression takes a few days, so don't worry kalau it doesn't happen exactly on CD15, every cycle can be a bit different. Just keep testing until you see that obvious positive, and then you know you're in the fertile window!