This is a test for detecting a particular hormone known as Human Chorionic Gonadotrophin (HCG). In the first few days after conception, the egg implants itself in the wall of the uterus (womb). The developing placenta secretes HCG (hormone) in increasing quantities as the pregnancy progresses.
Unlike the urine test, the blood test can detect pregnancies as early as 7 days after fertilization (even before a missed menstrual period!). Mothers with twins will normally have correspondingly higher HCG levels.
This test is performed from a venipuncture specimen.