well if ur gf feels that she is missing her periods just make her to eat lot of hot food such as pineapple, papaya, or make her to have lot of concentrated black coffee these things will heat up her body n get her periods... even i m in a relation since last 4 years n always had unprotected sex with my boyfriend anytime we wanted never calculated the time n date... but never got pregnant well it was me luck that i never conceived but u cant be 100% sure for it try to avoid sex during her periods coz it may create infection for both n yes black coffee 3 times a day may help alot prefer condoms or pills n for married women better use copper T to enjoy unprotected sex
orange juice can be used too