Possible to be pregnant yet have a negative 5 weeks after unprotected sex and an abnormal period (light, skipping days and then mid cycle bleeding?

Pregnancy unlikely: Home pregnancy tests are very accurate and not affected by foods, herbs, common illnesses, sexually transmitted diseases, or medications. A correctly done urine 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 still worried, one can repeat one test, one week later.