What Causes Delayed Periods?

i had sex with my girl friend, 10 days ago, that is onthe 24 and 25th of July. Her menstrual period did come in hthe last week of previous month(June). Untill today(2nd August), her period is not coming. We had conducted a HCG card pegnancy test at home, and we found it negative. But, its not sure if she is pregnant. We are no ready for the pregnancy. Please advice what is the safest option as of now to stop the pregnancy. Please help