You can test for pregnancy when your period is delayed by one day. The most accurate way to detect pregnancy after a one-day delay in your period is to take a blood test for HCG, a protein hormone secreted by the placenta, which begins to secrete HCG on the seventh day after the union of sperm and egg, when the fertilized egg begins to settle, that is, on the eighth day after the formation of the fertilized egg, or at the latest on the tenth day after sexual intercourse, you can take a blood test for HCG to clarify whether you are pregnancy. If your period is delayed by one day, testing for pregnancy by early pregnancy test or pregnancy test is prone to false negatives. The HCG can only prove whether the pregnancy is intrauterine or ectopic, and it is recommended to have a gynecological ultrasound after 10 days of delayed period to determine whether the pregnancy is intrauterine or ectopic.