If a woman is already pregnant a morning after pill is unlikely to have any effect on the foetus/embryo.
If you got pregnant while on birth control again it isn't likely to harm the growing child, but don't rule out the possibility of birth defects. Same as if you started birth control if you were already pregnant.
I believe in .U.S that before you get birth control (pills/shot) you have an orientation and exam if it is your first time on birth control. I understand they will ask if there is any chance you could be pregnant and test for it.
Scaredbaka I am going to post you get another link to a site found by hotasfrick, if you care to read any of it it does answer most of your concerns.
Http://www.Fpaneo.Org/news.Asp?Id=2

I have heard that birth control pills are like the morning after pill and can cause abortion if u take a lot of them at once but I dont know.
Meagan

Some brands can be used that way under a doctors direction - however it is not advisable due to the high level of chemicals - effectively an overdose

http://www.Fpaneo.Org/story.Asp?Id=230

what if you take an ecp and aren't aware you're pregnant?


Q: "what would happen if you took the morning after pill when you were already pregnant but were not aware?"

a: "if a woman is already pregnant and takes the morning after pill, the pregnancy will not be harmed."