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Since your girl friend is 3 days delayed please wait for another 3-4 days and when she is a week delayed if she does not get periods please get her urine tested for pregnancy
and consult a Gynaecologist if she is pregnant the doctor will help or advise regarding abortion
or if the test is negative she will prescribe medicines for periods to come.I hope this solves your query for now as you seem to be very much worried.Please you as well as your girlfriend understand that other than pregnancy even severe mental stress
is also a reason for delayed menses so relax since the mistake has happened do not stress yourself and hopefully she will get periods soon.