A study released in the October 10, 2001 issue of JAMA (The Journal of the American Medical Association), says that these tests may not be as sensitive as was previously thought, but not for the reasons you might believe at first.
Home testing kits usually instruct women that they can be used as soon as their first day of their period is due, or the day after it is due. There are even tests available now that claim to be effective in detecting pregnancy 3-4 days prior to the day your period is expected.
These test kits work by detecting the hormone human chorionic gonadotropin (hCG) in the urine of the woman. The cells of the trophoblast will secrete hCG in amounts that grow exponentially after implantation occurs. The hCG is then excreted in the urine and used for testing. Test kits should pick up a pregnancy the third day after implantation, which is separate from the first day of your period.
While the test kits are accurate in saying that they can detect very minute amounts of hCG in the urine, many kits report being able to detect between 15 and 100 mIU/ml of hCG, that isn't where the problem in detection lies. Using the first day of your last period to predict when implantation occurs is the problem.
Delays in ovulation or implantation may occur more frequently that we had previously believed. In this study 10% of the pregnancies were not detected on the day that the menstrual period was expected, despite the fact that even more sensitive testing was used, meaning that there many false negatives. This study did not include women who have irregular periods.
The instructions in some pregnancy testing brochures states that one is not likely to be pregnant or is in fact not pregnant if the test is negative. This assumption can be harmful. If you take a pregnancy test and the results are negative, but your period does not come as expected, it is advised that you wait a week and retest for pregnancy. If the results are still negative it is probably wise to report to your doctor or midwife for a physical exam to rule out or confirm pregnancy or other issues that may be causing a delay in your period.