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It is seen that usually after the emergency contraception , the withdrawal bleed should start within 7 days. If she hasnt had her menses for last 3 weeks now then it is imperative to rule out pregnancy . please get a urine pregnancy test done . if the test is negative then she can wait for another week for her menses to resume as it could be a side effect of plan b.
But first pregnancy has to be ruled out to ascertain contraceptive failure.
I hope i have answered your query in detail.
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