I have done sex with my girlfriend after her periods is there any cause of pregnancy once I have done sex before her periods is there cause of any pregnancy I did'nt use any condoms during sex... please clear my doubt if so what should I do to prevent her from pregnancy

Any time you have sex you risk pregnancy. Since you didn't use any form of birth control, the chance of pregnancy increases. The only thing you can do now is hope and pray that she gets her period when it is due.