god damnit i just posted a huge long post but bos screwed up. anyways, like i had already said, there are many different types of pill, HOWEVER, they all stop you from ovulating. It's not an "abortion" affect - there would be issues in manufacturing and selling such a drug (and no, the morning after pill does not have an abortion effect) - you do not ovulate, meaning there is no egg, meaning abortion can't possibly happen for there's nothing to abort. The pill tricks you into thinking you are pregnant, which is why you need to take 21 pills before you take a week off - you need to have taken a "cycle" to make sure you don't iovulate. What happens is for 21 days your body thinks oh I'm pregnant, so you don't produce or release any eggs because well, you don't need to if you're pregnant. Meanwhile, the uterus lines itself preparing for this new baby, but then you take the week off and your body thinks oh noes, im not pregnant, and sheds the lining causing your period. At no point is an egg produced or released. That is an effect that one of the ingredients in the pill causes - the ingredient that is in all the pills. The one that differs or is absent from some pills, is the one that *MAY* change the lining of the womb, making it less likely for a fertilised egg to implant, say in the case during the 21 days you forget to take a pill and you being ovulating, causing an egg to be released, it tries to protect you that way.
But either way, ovulation is not meant to occur on the pill, no matter which pill you are taking, hence you're saving your eggs.