When you are using ovulation prediction kits (or devices that measure luteinizing hormone), you can usually expect ovulation to occur the day after your first positive result. This is because (LH) luteinizing hormone, the hormone measured by OPKs and the hormone responsible for triggering the rupture of the ovarian sac, typically surges the day before ovulation. In some cases, however, you will see ovulation on the same day as the positive result. There are a few factors that can help explain how this happens.
First, remember that when you are reading your OPK, you are seeing a snapshot of your LH surge. You do not really know whether you are seeing the beginning, peak or trail of the surge. What you know is that your LH is at a level that is detectable as a surge by the kit. It may have started to surge soon after your test the previous day and be starting to trail, or it may be just starting to surge. In a case where it looks on the chart as if ovulation occurred the same day as the surge, here is what may have happened:
1. LH started to surge during the night or early in the morning and was detected by your OPK in the early afternoon.
2. Ovulation occurred sometime in the evening or night, several hours after your positive OPK, possibly several hours longer than when the LH actually started to surge.
3. Your temperature is up in the morning, indicating ovulation for the previous day, the same day as the positive OPK, even though many hours have elapsed between the LH surge and your temperature rise.