Pregnancy tests work by checking your urine or blood for a hormone called human chorionic gonadotropin (hCG). After a fertilized egg attaches to the uterine wall, the hormone hCG is actively produced by the chorion tissue (eventual placenta). As a result, the corpus luteum - a mass of cells in the ovary essential for establishing pregnancy - continues progesterone secretion until the placenta is able to produce estrogen and progesterone itself. In contrast, during a normal menstrual cycle without egg fertilisation (that is, no pregnancy) the corpus luteum decays after about 2 weeks.
The concentration of hCG increases more slowly in the urine than in the blood. This means that a laboratory blood test can already observe increased levels of hCG 6-8 days after conception. This is 4-5 times faster than a home urine test, so it makes sense to wait until about 12-14 days after conception before doing a home pregnancy test. This coincides with the first days of your missed period and 4-5 obstetric weeks of pregnancy.
In a home pregnancy test, pregnancy is indicated by a beta-hCG value greater than 5 ppm per ml of urine. When choosing a home pregnancy test, it’s important to take the sensitivity into account. A smaller sensitivity number means the test will show a positive result with a lower hCG content in the urine. This means that if you are prone to false positives, choose a test with a higher sensitivity value.