A pregnancy test that diagnosed pregnancy the morning after conception would make a fortune. Whether they're hoping for a "yes" or a "no" answer, millions of women each year anxiously await the answer results of a test that tells them whether or not they're pregnant. As of 2010, it's not possible to accurately test for pregnancy until around the time of the first missed period, two weeks after conception. Some ways to test for pregnancy are more accurate than others.
Vaginal Ultrasound
The most accurate test for pregnancy is visualization of the embryo or, before the embryo is visible, the gestational sac, on vaginal ultrasound. Usually visible within one week after a missed period, or by three to four weeks after conception, a gestational sac is proof that an embryo has implanted, Dr. Joseph Woo of Hong Kong reports on his website, Obstetric Ultrasound: A Comprehensive Guide. While not proof that the pregnancy is viable or will continue, vaginal ultrasound does accurately document proof of conception.
Blood Tests
Blood tests that measure human chorionic gonadotropin, often called hCG, very accurately diagnose pregnancy around the time of a missed period. HCG, produced by the newly developing placenta, registers a positive result between seven and twelve days after conception, the American Pregnancy Association explains. Blood tests give quantitative results, meaning they measure the exact amount of hCG in the blood. Quantitative tests help track the normal rise of hCG blood levels. In the first few weeks of pregnancy, hCG blood levels double every two to three days, Advanced Fertility Center of Chicago states. Blood tests must be ordered by a physician and cost more than urine pregnancy tests.
Urine Tests
A number of commercial home pregnancy tests sold in pharmacies without a prescription measure urinary levels of HCG and often give positive results as early as a few days before the first missed period, depending on the sensitivity of the test brand, Brown University Health Services reports. False negatives more commonly occur with urine tests than blood tests, because hCG may be present but in too low of a concentration to register positive on a particular test brand.
Urine tests are qualitative, meaning they don't give an exact reading on the amount of hCG present, but register simply as positive or negative, says the American Pregnancy Association. A negative test doesn't rule out pregnancy; if menses doesn't start within another week, taking another pregnancy test may register a positive result.
