To answer this question, you need to understand the basics. When conception occurs, the female body begins to produce a hormone called the "pregnancy hormone". But this does not happen immediately, but after a while. The strip is colored due to the fact that the test contains a special substance that reacts to the presence of this hormone. So if it is in the urine, the test shows two strips. Everything is logical and elementary. The testing mechanism is really simple, then why are there failures in it?
But immediately after sexual intercourse, the level of hCG (the same "pregnancy hormone") does not increase. After the sperm fertilizes the egg, the concentration of the hormone will not change. So even in such a situation, the test, which was done in the morning after sex, will not show anything. Hormonal levels begin to change only 6-10 days after the egg has been fertilized. And at the same time, even in the early days, if the sensitivity of the test is low (and this applies to most products from pharmacies and stores), it may not show the correct result.
If we talk about the most common tests, then their sensitivity is 25 mIU / ml. This means that they will react to the onset of conception only about two weeks after it actually happens. That is, you should ideally use such a test a couple of days after the delay. In the first days, you will need to wait a few minutes, then the strip will be colored. And then the level of the pregnancy hormone will increase so much that even the most insensitive test will accurately react to it - and most likely will show the correct result and very quickly. Some women use several tests in a row in the early stages, but this does not affect the accuracy of the result.
There are tests with a sensitivity of 10 mIU / ml, which allow you to determine pregnancy at a very early stage. They can be used a few days before your period is due. You can buy such a test at a pharmacy, just ask a pharmacist for it.