Pregnancy Test

All pregnancy tests measure the amount of human chorionic gonadotropin (hCG), the pregnancy hormone, in your body.  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 chemical tests for pregnancy 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 published detection thresholds of 20 mIU/mL to 100 mIU/mL, depending on the brand. Qualitative blood tests generally have a threshold of 25 mIU/mL, and so are less sensitive than some available home pregnancy tests. Most home pregnancy tests are based on lateral-flow technology.

 
Home pregnancy tests are urine tests. They detect the amount of hCG in your urine, but only when it reaches a certain level. Some home pregnancy tests are more sensitive than others. More sensitive tests may be able to detect low levels of hCG a few days before your period is due. 

However, whether you use an ordinary or a more sensitive home pregnancy test, if you use it too early in pregnancy the amount of hCG in your urine may not be high enough for a positive result. Almost all urine pregnancy tests will give you accurate results if you test at around the time your period would normally be due; about two weeks after you ovulate. 

A test may be negative for several reasons: you may not be pregnant, or you may have ovulated later than you thought (and so not be far enough along for the test to detect your hCG). In a small number of women, hCG levels can be very low and this may also affect the outcome of pregnancy tests. 

The blood test for pregnancy measures the amount of hCG in your bloodstream, not your urine. Blood tests can measure much smaller amounts of the hormone, and so can detect pregnancy earlier than urine tests, usually about six to eight days after ovulation. However, these are only available through your doctor who is unlikely to offer one unless you have pressing medical reasons to know quickly whether or not you are pregnant. 

ACCURACY

The accuracy of a pregnancy test is most closely related to the day of ovulation, not of the act of intercourse or insemination that caused the pregnancy. It is normal for sperm to live up to five days in the fallopian tubes, waiting for ovulation to occur. It could take up to twelve further days for implantation to occur, meaning even the most sensitive pregnancy tests may give false negatives up to seventeen days after the act that caused the pregnancy. Because some home pregnancy tests have high hCG detection thresholds (up to 100 mIU/mL), it may take an additional three or four days for hCG to rise to levels detectable by these tests — meaning false negatives may occur up to three weeks after the act of intercourse or insemination that causes pregnancy.

A systematic review published in 1998 showed that home pregnancy test kits, when used by experienced technicians, are almost as accurate as professional laboratory testing (97.4%). When used by consumers, however, the accuracy fell to 75%: the review authors noted that many users misunderstood or failed to follow the instructions included in the kits. Improper usage may cause both false negatives and false positives.