Pregnancy tests are very sensitive and can be positive (show you're pregnant) even before you miss a menstrual period. Most tests are positive 7 to 10 days after you conceive; this includes blood, urine and home pregnancy tests. Most doctors recommend you wait until you miss your period before having a test. This saves you money and emotional energy.
Quantitative HCG Test
A quantitative HCG (human chorionic gonadotropin) test is a blood test done in the first trimester if there is concern about miscarriage or ectopic pregnancy. The test measures the hormone HCG, which is produced early in pregnancy and increases rapidly. Two or more tests done a few days apart are more useful than one test because the change in the amount of the hormone present is significant. An ultrasound is also often done when a quantitative HCG test is ordered.