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You have not mentioned if her periods are regular or not. Periods can delay upto 10 days even due to emotional stress
, any change in health. If her periods are delayed and she is not pregnant it can be because of hormonal
disorder which can be caused by ovarian cysts also. You can ask her to take Pregnancy
test as if she is pregnant it will become positive in 7-10 days of unprotected sex. Use condom to prevent unwantd pregnancy and STDs.