According to the companies that manufacture the pills there are no side effect having to do with getting pregnant later on. The bigger question is regarding other side effects and possible risks.
These pills basically trick your body in to thinking that it's pregnant by raising hormonal levels. The companies claim that the change in breast size is usually permanent and that no milk is created as opposed to pregnancy. The fact of the matter is that there hasn't been much testing done in this field that can point in one direction or the other.
I personally don't believe in messing around with the body's hormonal balance and I think till more substantial research is done you should act with caution. Don't forget, even regarding pregnancy, sometimes side effects are discovered after a while that a product has been on the shelves. Good luck.