How soon can I test? Well, first you gotta know how the test works. Briefly: the home pregnancy test (HPT) works by detecting a hormone called human chorionic gonadotropin (hCG.) When an egg is fertilized, it takes about 6 to 12 days to implant in the uterus, and then it begins to secrete hCG. It takes another day or two for enough hCG to build up and make its way into your pee. So, generally speaking, the earliest you can expect a positive HPT is 8 days past ovulation ("DPO") and the latest would be about 16 DPO. It is possible to get a positive test earlier or later, but not as common. In my experience, if you use a sensitive "early" test (anything that detects 25 mIU/mL of hCG or less) you can test around 12-14 DPO and feel reasonably secure with your results (the later you test, the more secure you can feel in its accuracy.)
If you do not chart or monitor your fertility in any way, and you think you may have conceived, you should wait 19 days (or longer) after having sex to test. Why 19?! Sperm can live approximately 5 days in the proper environment; if the sperm managed to live until you popped an egg, the actual conception could have taken place several days after the sex act that got the sperm there. To make sure you're not testing too early, assume the sperm had very long lives (5 days is considered a very long sperm life) and that conception occurred then. Then allow 14 days for the embryo to implant and develop enough hCG to show up on a pregnancy test (96% of pregnant women will test positive by then.)
Note: some HPT's say "test 5 days before your period is due!" or some such thing. Please note they are assuming a textbook 28-day cycle, with ovulation on Day 14. "5 days before your period is due" thus really "translates" to 10 DPO.
Essentially, you should remember that your high school biology textbook was WRONG. NOT all women have 28-day cycles and NOT all women ovulate on Day 14. For more info on this type of thing, see the Fertility Info section.
Here's an example. This is the textbook-perfect 28-day cycle with ovulation occurring on day 14 (in pink.) 10 DPO and later are reasonable days for testing (in green.) This is the cycle the HPT companies have in mind when they say "test 5 days before your period is due!"
Well, not everyone's body works this way. Here's another 28-day cycle-- but this one has ovulation occurring a little bit later, still well within the normal range. Watch what happens to your (green) testing opportunities!
All of a sudden, "5 days before your period is due!" is way too early for you to expect a positive test! (I tend to ovulate later myself, so by the time the "gold standard" of testing at 14 DPO is applicable, my period is already 3 days late! I needed to use an "early detection" test on the first day of my missed period. A "regular" test would probably have been negative-- even though my daughter is living proof that I was, indeed, pregnant
Of course, it works in reverse, too. If you're an early ovulator, you're lucky! Check it out:
In this scenario, you could test even sooner than 5 days before your expected period.
Ultimately, it's important to remember that implantation is a variable thing. You may be able to detect a pregnancy very early on, but then again, you may not! If you can't, don't despair.