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Both are safe periods, but more reliable is the last 7 days before the next period, since ovulation has already occurred and the ovum is already in the uterine cavity
, where it cannot be fertilized. Fertilization occurs in the Fallopian tube
and the fertilized egg then travels to the uterus
and gets embedded there.
All this is true for regular period. In case of irregular period one cannot be certain that she will get the period immediately after 7 days.