Expert: Norman Solberg - 2/24/2009

Question
Hello =) I have a couple of questions about the morning after pill.  I have an appointment next week to get back on birth control, but my boyfriend and I were less than careful this past weekend.  My first question is how quickly does fertilization occur?  If I understand correctly, the primary way Plan B works is to delay ovulation to begin with.  I don't track my cycles as precisely as some, but I have a pretty regular cycle, so I have a general idea of when I MAY have ovulated.  I would consider the time we had sex to be very high risk if I'm right, though I know it's not always an exact science =).  If there was already an egg waiting, is it possible fertilization could have occurred before I took the pills?  And if so, how effective is Plan B at preventing implantation?  The paperwork included says it may prevent a fertilized embryo from implanting, but it seems like the primary purpose of the morning after pill is to prevent an egg from being there in the first place.  It seems like a long shot, but I'm just trying to make sure!

Answer
As you say, it's not an exact science.  Plan B works better the earlier you take it.  Sperm need to undergo capacitation after ejaculation, which takes about 7 hours.  So fertilization could occur 7 hours after intercourse if the egg was in the tube.  Plan B does inhibit ovulation, but also seems to work even after ovulation has occurred, although the exact mechanism is unclear.  But in general Plan B is almost 90% effective in preventing pregnancy, and more effective if taken within the first 24 hours.  So whether fertilization could have taken place depends on how soon you took the plan B.  In any case, it works well, and it's unlikely you're pregnant.  However only time will tell.  When you get your period you'll know for sure.