Hiya
You can use Ovulation detectors as a Pregnancy test - But you would get a positive reading on the Ovulation test if you were actually ovulating, so its best to use a HPT.
The reason you can use ovulation predictors as a pregnancy test, because the hormones they both detect (Lutenising Hormone in Ovulation detectors and HCG in Pregnancy tests) are both molecularly the same, so an ovulation detector will show a positive result if it detects the presence of either HCG or LH.
Hope that makes sense.