A blood test is the most reliable way of measuring hCG levels during pregnancy. This is so because though hCG is also present in urine (hence, the home pregnancy tests that sample urine to determine pregnancy), the density of the hormone in the blood is much higher than that of urine. Blood tests can detect levels between 5 and 10 mIU/mL versus the 20 mIU/mL that at-home pregnancy tests can detect.