What Causes Delayed Period After Having Unprotected Sex?

ls need your help i had sex with my wife 20 days back without any protection. her last period was on 13th april, but her 2nd periods is not yet started.what to do?we are married 20dayd back and she is not willing to consult a doctor.pls help me. She is 28 years and 5.5ft 61kg
Madhu, MDY