About 4 months ago, I switched from Alesse to Yaz. At the beginning of my most recent pack, I accidentally began using an old Alesse pack that I had forgot to throw out instead of Yaz. I discovered this 4 days in to the pack, and immediately switched back to Yaz as per my doctor's orders. My partner and I have been using the withdrawal method this month just to be sure, but I am worried that some ejaculate may have still come in contact with the outer parts of my genital area. I would like to know the possibility of pregnancy based on this scenario, and whether I should go ahead and take Plan B.