By the 10th day from ovulation — usually the first day of your missed period — there is enough hCG in your urine for the store-bought pregnancy tests to detect it. Blood from your period will not impact whether or not there is hCG in your urine, so it wont impact your test results.
Can blood contaminate a pregnancy test?
Most of the time, false positive results come from improper testing, especially while using at-home testing. Certain medications can interfere with pregnancy tests. Sometimes urine can be contaminated with blood or large amounts of protein, which can also lead to inaccuracies in test results.
Can a pregnancy test be positive if not pregnant?
Its possible to have a positive pregnancy test even if you arent technically pregnant. This is called a false positive. Its sometimes caused by a chemical pregnancy. A chemical pregnancy occurs if a fertilized egg, known as the embryo, is unable to implant, or grow, very early on.