Last resort is such an odd term to use though, you know? I assume you mean after considering keeping it, adoption, and giving it to the father, and if none of those work than aborting it, but it's still such a sticky situation. Why should a woman be forced to be pregnant for 9 months of their life if they have no interest in keeping the baby? Life is short, too short to be carrying around a fetus that you have no intention of keeping for the better part of the year. How was your 2010? "I spent it puking, going on crazy eating rampages, gaining 30 pounds, and having mood swings, and then I gave the baby up for adoption." That doesn't sound fair.
This raises the whole issue of "accepting the consequences for your actions," which I think is the biggest bullshit pro-lifers throw at you. If you can fix a mistake quickly after it was made, why shouldn't you go for it, especially if the mistake will cause you 9 months of pain, mood swings, weight gain, and all sorts of other unwanted side-effects. "Sorry, you had (probably but not necessarily) unprotected sex, now you have to have a baby." That hardly seems fair at all considering we have the technology to abort a baby safely.