Well i just read this :It explains that testing for hCG is not the same in all women. In 10 percent of pregnant women, the embryo does not implant until after the first day of a missed period (and again, hCG is not produced until the embryo implants in the uterine wall).
"Until implantation, it doesn't matter how sensitive the test is," CR says, "You can't detect the pregnancy before it's producing the stuff that you're measuring, which is hCG."
Even pregnancies that have implanted may produce too little hCG for many at-home tests to detect, especially only a day or two after a missed period and when read after the manufacturer suggested waiting period specified in test packaging.
"Some kits improve in detection when read after a wait of 10 minutes, but waiting longer than that may produce a negative result that looks faintly, misleadingly positive," CR reports.
Moreover, about one-third to one-quarter of pregnancies implant for a short time, then fail. "That leads to a transient rise in hCG, which can extend for as long as two days after the day of the missed menstrual period."
Consumer Reports advises, "If you're comfortable waiting, a sensitive test taken a week after your period is overdue will almost certainly give you accurate results. If you elect to take the test as early as the day after you've missed your period, remember that a negative result isn't 100 percent certain. And a positive result may mean either a viable pregnancy or a pregnancy destined to end shortly after it began. With either of those results, you should plan on testing again a week later, just to be sure."
its from a CBS study on pregnancy tests..so maybe i'm not completely out