Both tests check for the pregnancy hormone called hCG. The production of human chorionic gonadotropin (hCG) begins when a fertilized egg attaches to the wall of the uterus, around 6 days after becoming pregnant.
As hCG builds in a woman’s body, it enters her bloodstream and urine. A blood test can accurately detect hCG as soon as a week after ovulation. Urine pregnancy tests detect hCG later, a day or two after the missed period.