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1. Usually the possibility of ovulation is less during withdrawal bleeding, so the need for emergency contraception is less. But as you took the pill already, if at all there is any possibility of pregnancy, it can help.
2. Because of high dose of hormone in it, it can affect your periods. Sometimes after taking emergency contraceptive pill, you may see withdrawal bleeding after 7 to 10 days.
3. For accurate result better to test three weeks after having unprotected intercourse.