When choosing a pregnancy test, they vary wildly in the levels of hCG the tests detect. And the lower the level listed on the kit, the more sensitive to the presence of hCG it is, not the other way around like many believe.
For pregnancy testing, the concentrations of hCG are reported in milli International Units (mIU) or amounts equal to 1/1000th of an IU per milliliter. In simpler terms that you will find listed on the test box, pregnancy tests with a sensitivity of 20 mIU/ml are more sensitive than tests with 50 to 100 mIU/ml.
With 20 – 25 mIU/ml tests, this means the test can determine pregnancy much earlier than the ones in the 50 to 100 mIU/ml range. If you are past your period due date, choose a test in the 50 to 100 mIU/ml range, as they are usually much less expensive.
However, if you are trying to conceive and want to know sooner, you might find it worth the expense to buy a pregnancy kit in the 20-25 mIU/ml range. You can begin testing for pregnancy as early as eight days after you believe conception might have occured. Also keep in mind that conception can take place up to three days after intercourse
That means you can begin testing accurately as early as eight days after conception. Conception may take place several days after date of intercourse, keep this in mind when you plan the date for pregnancy testing for each cycle.