What is the % that a pregnancy can result if a woman is a week away from starting their menstrual cycle & have unprotected sex then take plan b pill?

Depends: Depends on when you ovulate (varies) and how soon afterwards you used plan b. If you have a regular cycle, then not likely to be ovulating on cycle day 21, assuming menses start on day 28. If Plan B taken within 12-24 hours , then likelihood is low, around 5%. If taken 24-72 hrs later, then around 10% chance. Condoms are less stressful!