All pregnancy tests, whether taken at home or at a doctor's office, detect the level of human chorionic gonadotropin (hCG) in either urine or blood samples. hCG is a hormone created by the developing placenta. hCG builds up rapidly in your body shortly after the embryo attaches to the uterus. It can be detected by a home pregnancy test 3 to 4 days after a missed period. Some tests can even detect the hormone several days before your missed period.
Be sure to read the directions for the pregnancy test very carefully. There are various ways the urine sample is collected and tested. Most commonly, you will urinate directly onto a testing stick and wait 3 to 5 minutes for the results to appear.
Urine-based home pregnancy tests are very accurate. Positive results almost always mean pregnancy. False-positives are rare (this is when the test shows that you're pregnant but you're not). A positive result does not guarantee that the pregnancy will succeed. It also does not show that the pregnancy is located within the uterus.
On the other hand, a negative result does not mean that there is no pregnancy. Since some tests detect a lower level of hormone than others, the type you choose may not detect a pregnancy in its early stages. If you suspect you are pregnant but your test comes back negative, wait a few days and take another test.
When a pregnancy test shows a positive result, contact your health care provider to schedule a prenatal evaluation and exam.
Pregnancy tests performed by a health care provider using blood samples can detect a pregnancy 1 to 2 days after implantation -- a few days before a missed period. Blood tests can pick up very low levels of hCG and will show results almost immediately.
All pregnancy tests are fundamentally the same -- they measure the amount of hCG in your system and produce positive or negative results based on that measurement.
Reviewed By: John D. Jacobson, MD, Professor of Obstetrics and Gynecology, Loma Linda University School of Medicine, Loma Linda Center for Fertility, Loma Linda, CA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.