Pregnancy tests look for hCG (human chorionic gonadotropin) which is excreted in pregnancy. You can detect hCG in either blood or urine pregnancy tests. Which type of pregnancy test your doctor or midwife requests will depend on what they are looking for with your pregnancy.
If it is merely to confirm that you are pregnant, a urine pregnancy tests or home pregnancy test will suffice. If your practitioner has a reason to suspect multiple pregnancies, ectopic pregnancy or a miscarriage, a blood pregnancy test is more often used. Sometimes you will have these blood tests repeated to watch for a rise in the hCG levels.
The rate of rise for hCG in pregnancy is that it nearly doubles about every 48 hours in the first 30 days after implantation, about 7 weeks gestation, though this can vary. After that, the rate of rise begins to slow. hCG levels peak at about 8–10 weeks, then slowly decrease until they level out at about 20 weeks and stay constant for the rest of pregnancy.