I last saw my period on the 24th of june, i had unprotected sex(withdrawal method) on the 12th of july then i was expecting my period on the 19th according to my flow calendar, now it says i am 3 days late. i tested for hcg yesterday, result negative?

Tests are accurate: Today's pregnancy tests are very accurate and not affected by foods, herbs, common illnesses, sexually transmitted diseases, or medications. A home pregnancy test is reliable by 3 weeks after sex (3 weeks after when conception might have happened) or 1 week after missing a period. If done sooner, a positive result is likely real, but a negative result may be due to testing too soon.