My nurse said opk can detect hcg in the system after the trigger. does this also mean opk can detect pregnancy on the day period is due.

Cross reaction: An OPK is meant to detect the hormone lh which surges at the time of ovulation. A urine pregnancy test is meant to detect hcg, the hormone of pregnancy. However, both HCG and lh are very close relatives chemically and that's why the OPK can pick up a pregnancy and the urine pregnancy test can pick up an ovulation surge. You should still use each test for its specific purpose.