Male condoms are 98% effective if they are used correctly and every time you have intercourse. If not used perfectly every time, they are only 82% effective. It may be helpful to tell your girlfriend that you will go with her to have a pregnancy test and that you will also be there for her whatever the results might be. Tell her that if she is pregnant, it’s important to know sooner rather than later. Stress can definitely play a role in delaying a girl’s period but it is still best to do a pregnancy test to make sure.