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If you had protected sex there will be less chance of pregnancy
if everything was proper.If your girl friend has missed periods & you are worried you can get a pregnancy test done to rule out if pregnancy is there .If the test is negative tell her to consult a Gynaecologist who will examine her & prescribe medicine to regularize her period.As periods can be missed due to hormonal
imbalance & even stress
I hope this answer is helpful to you.