After reaching vagina, sperms are able to fertilize upto 48 hours. After that, they're dead. So if ovum was already there, the chances of pregnancy is almost sure provided you both are normal. Pregnancy can be detected just after the fertilization. I mean after having intercourse, you can detect pregnancy minium after 2 days through pregnancy kit.
Now, if the girl has been already fertilized, then abortion through tablets are almost impossible. But there comes some expensive progesterone killing tablets but they have lot of side effects like excess bleeding or pain. But a better option of abortion is vacum tube method which is relatively safe.