Does having irregular period affect blood preg test result after 20days of sex? why some say that to wait for the longest cycle bfore taking test?

No. Test is accurate: Pregnancy tests are very accurate and not affected by foods, herbs, common illnesses, sexually transmitted diseases, vaginal spotting, or medications. A test gives a reliable result 3 weeks after sex (3 weeks after when conception might have happened) or 1 week after missing a period. If done sooner, a negative result may be due to testing too early such as less than 2 weeks after conception.