Emergency Contraception: Understanding Plan B and Effectiveness

Question
sorry my topic is not abortion. the other experts i have written to have not answered so i am taking a chance of asking you because you answered my last question about abortion very quickly and very clearly. my last period started december 16, my cycle is usually 28-33 days long. i had unprotected sexual contact on december 27 and 28, and on the evening of december 28 i took the first dosage of plan B emergency contraception pills (got the dosage from the princeton ec website). i continued with the second dosage 12 hours later, at noon on december 29. however i had unprotected sex december 29 in the afternoon. is there a chance of pregnancy given my situation or will the plan b pills prevent it since i have not had sex since then. please help me. i want to know what i should do.

Answer
Dear Anne,
Hopefully you are fine, and did not get pregnant.  There is no way to know for sure what the chances are that you did get pregnant.  Even in the best of circumstances (taken on the same day as unprotected sex), Plan B is only about 90% reliable.  You can try taking a pregnancy one week after the incident, and if it is negative, you are probably ok.  You may want to look into a better form of birth control.  
I hope this is helpful,
Doctor Joan