Will a pregnancy test give a reliable result 3 weeks after intercourse?

Yes, three weeks after intercourse, the pregnancy test should already show a reliable result. Pregnancy tests detect the hormone hCG in the urine, which begins to be produced after the fertilized egg implants in the uterus. This usually occurs about 10-14 days after fertilization. After three weeks, the hCG level is high enough that a pregnancy test, especially one with high sensitivity, should give a reliable result. If in doubt, I recommend consulting a gynecologist.

A pregnancy test can be purchased at an online pharmacy:

https://www.gdziepolek.pl/produkty/94711/baby-boom-test-ciazowy-kasetowy-test/apteki

I encourage you to read:

https://www.gdziepolek.pl/artykuly/test-ciazowy-jaki-wybrac-i-kiedy-zrobic

To @Magdalena:

Based on your menstrual cycle, which lasts 26 days, and the date of your last menstrual period (17.08), it can be estimated that ovulation took place around 28.08. Fertile days cover the time from 5 days before ovulation to 3-4 days after it, which means that the intercourse that took place on 31.08 fell on the fertile days. During this period, there is an increased chance of pregnancy because the egg only lives up to 24 hours after ovulation, and sperm can survive in a woman's body for 2 to 7 days.

Hello. I had my period on 17.08, cycles 26. I had intercourse on 31.08.

One line is visible on the test, which means that you are not pregnant. The test result is read from a few to several minutes after it is performed, later the result may no longer be reliable. As I wrote - the most reliable image will give you beta-bCG from blood and a visit to the gynecologist.

Here's an example.

If I were you, I would do a beta-hCG blood test - it can detect pregnancy as early as 6-8 days after ovulation, because it measures the level of the hCG hormone (chorionic gonadotropin) directly in the blood. This is a more sensitive method, it can be performed in laboratories, e.g. Alab, synevo, diagnostics.