Pregnancy tests detect the presence of the pregnancy hormone (HCG) in both blood and urine. HCG begins to be secreted by the trophectoderm of the embryo after implantation and passes into the blood and urine.
The concentrations of this hormone in urine are a little lower than in blood, especially in the first days of its possible detection, so if we perform a test too early it is easier to detect it in blood than in urine.
Nowadays there are very sensitive urine tests in pharmacies that detect already very low amounts of HCG around 12-25 IU of HCG. Even if a woman's cycle is irregular, HCG begins to be detected between 10 and 15 days after the possible conception, therefore, if it is known approximately when the possible conception occurred, the test can be performed about 15 days later.