Could an emergency room urine preg test give a false negative 21 days after sex. i thought i had a period it was light and took the test week after ?

Very reliable at 21d: Urine pregnancy tests are very accurate and not affected by foods, herbs, common illnesses, sexually transmitted diseases, or medications. A urine test gives a very accurate result by 3 weeks after sex (3 weeks after when conception might have been). If done sooner such as less than 2 weeks after sex, a positive result is likely real, but negative result may be due to testing too soon.