If you are pregnant, you may be aware that the early tests, that is, before the first missed period are not very accurate. They may give you false negative results when you are actually pregnant. The answer to the question how early do pregnancy tests work is that it all depends on pregnancy test used. All the pregnancy tests in the market detect the presence or amount of HCG (human chorionic gonadotrophin) hormone.
The common home pregnancy test kits basically detect HCG hormone presence in the urine. The level of sensitivity of the pregnancy test kits varies. The lowly sensitive pregnancy tests many not detect low HCG levels and may give false negative result. On the other hand, the highly sensitive pregnancy tests kits may actually detect low HCG levels early, that is, about 4 days before the date in which you should have received monthly period if you were not pregnant.
The best and most highly sensitive pregnancy test kits can even detect presence of this hormone 7 days after conception. On its packaging, the pregnancy test sensitivity is described as mlU/ml. The sensitivity of the pregnancy tests ranges between 10 mlU/ml and 40 mlU/ml. A lower number means that the pregnancy test is more sensitive and can detect pregnancy earlier.
A test, too soon after pregnancy even with the most sensitive test may give false negative result because the HCG amount in urine may not be in sufficient levels high enough for the test to detect it. A false negative test in which the test shows that you are not pregnant, and actually you are can be very stressful. Most of the home pregnancy test kits in the market can provide accurate results if the test is done at a time in which the period will otherwise be due, that is about 2 weeks after ovulation.
There are various reasons why pregnancy results may be negative while you are actually pregnant. First, ovulation may take place later than expected. For most tests to show accurate results there should be period of two weeks after the ovulation and about 4 weeks after the fertilization.