Well, there is the obvious such as condoms and birth control, but if you had sex and then are worried there is the ec pill available at most health clinics. But it is only effective 72 hours after sex. Does that answer?

well abortion doesnt prevent pregnancy it ends pregnancy. If you want to prevent it then use double birth control like pills and condoms or dont have sex.
If you may already be pregnant then its a matter of keeping or ending pregnancy.
Unless int in the first 72 hours, then therein the morning after pill. The fertilized egg isnt even implanted in the uterus yet within this time frame so dont feel too bad.
Hope this helps...