If you think you're experiencing implantation bleeding instead of your period, you can try a blood test 4-5 days after the bleeding starts. This is when hCG levels are detectable. Your doctor would need to perform the blood test and they may advise you to wait until after your missed period to get the most accurate result. If you want to try an at-home pregnancy test, it's best to wait for 7 days after the implantation bleeding starts. If you do take a pregnancy test and it comes back negative, it's entirely possible that it was a false negative. Wait a few days and test again when hCG levels have had a chance to increase.