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If after the sexual contact you had taken Emergency contraceptive pill or Morning after pill within 72 hours of the act then possibility of pregnancy is remote.They are quite effective in pregnancy prevention when taken within the stipulated time.
The periods might be delayed due to the pill intake as it causes changes in the normal hormonal levels.Irregular period which might be delayed by a week is one of the common side effect of its use. Relax and wait for the periods.If they do no start even after 10 days of the expected date the consider visiting a Gynaecologist for evaluation.
Take care!