Note: Information provided on this page is for general education only, please seek medical assistance when in doubt
Pregnancy test is an attempt to determine whether a woman is pregnant. Pregnancy test is generally done either by sampling urine or blood. Of the markers that indicate pregnancy, the first of these markers -- human chorionic gonadotropin (hCG) -- was discovered in 1930 and produced by the trophoblast cells of the fertilized ova (eggs). While hCG is a reliable marker of pregnancy, it cannot be detected until after implantation and this however results in false negatives if the test is performed in very early stages of pregnancy.
The test for pregnancy which can give the quickest result after fertilization is a rosette inhibition assay for early pregnancy factor (EPF). EPF can be detected in blood within 48 hours of fertilization. However testing for EPF is expensive and time-consuming.
Most home pregnancy test kits are based on lateral-flow technology and look for the presence of the beta subunit of hCG, or human chorionic gonadotropin, in the blood or urine. hCG can be detected in urine or blood after implantation, which occurs six to twelve days after fertilization. Quantitative blood (serum beta) tests can detect hCG levels as low as 1 mIU/mL, while urine test strips have detection thresholds of 10 mIU/mL to 100 mIU/mL.
Obstetric ultrasonography can visualize the gestational sac as early as four and a half weeks of gestation and the yolk sac at about five weeks' gestation. The embryo can be observed and measured by about five and a half weeks. The heartbeat may be seen as early as six weeks, and is usually visible by seven weeks' gestation.
Accuracy of home pregnancy test kits when used by experienced are almost as accurate as professional laboratory testing while when used by general consumer has accuracy of about 75% which primarily is because of failing to understand or follow the instructions properly included in the kits.
False negative readings can occur when testing is done too early. Quantitative blood tests and the most sensitive urine tests usually begin to detect hCG shortly after implantation, which can occur anywhere from 8 to 10 days after ovulation. hCG levels continue to rise through the first 20 weeks of pregnancy, so the chances of false test results diminish with time. Less sensitive urine tests and qualitative blood tests may not detect pregnancy until three or four days after implantation and sometime may need up to three weeks after ovulation to detect pregnancy.
Additionally spurious evaporation lines may appear on many home pregnancy tests if read after the suggested 3 to 5 minute window or reaction time. False positives may also appear on tests used past their expiration date.
Some diseases of the liver, cancers, and other medical conditions may produce elevated hCG and can cause a false positive pregnancy test.