Blood testing is also used to test for pregnancy since a blood test can detect a pregnancy much earlier than a urine test. In this test hCG is measured in your blood stream. A sensitive urine test might detect 20–25 mIU/mL of hCG in your urine. But hCG is more easily detected in blood, where a test can measure levels as low as 5 mIU/mL of hCG.
Blood tests are great for determining pregnancy early, particularly if you are experiencing complications. Since hCG levels nearly double every forty-eight hours, you can do serial blood testing to check on a pregnancy's viability before the point where an ultrasound is useful. Ultrasound isn't of much use until you have about 1,500 mIU/mL of hCG in your blood, which occurs at about five weeks gestation.
Using blood hCG testing it is also sometimes possible to recognize an early miscarriage if the numbers aren't rising as quickly as they should, or if they are even falling. Blood tests can also help detect an ectopic, or tubal, pregnancy, because of abnormal rates of rise noted. If your hCG is rising very rapidly, this could indicate other situations, including that of a multiple pregnancy, like twins!