Question for Pro-Choicers (Man kills zygote)

I've never really spent much time thinking about the abortion debate, but when I do, I can't think of a solid argument for being pro-choice. I also believe logic rather than morals or religious beliefs can be used to form a strong argument for being pro-life (I state this b/c I've heard pro-choicers label pro-lifers as moral elitists).

I don't know if pro-choicers is the correct term, and I don't regularly follow the threads in this forum, so I apologize for using an incorrect term or if this topic has been frequently touched upon.

Anyways, this question is for those of you who don't believe that a fetus (or zygote, or whatever collection of cells after conception) is not the same as a person.

If a man punches (or gives a good Chuck Norris kick to) a woman's belly, and kills her zygote/fetus/etc soon after conception, how much of a punishment should he receive? How would his punishment compare to him killing an 8-month fetus, an infant, a child, etc?

Just to start things off, I'd assume those who are pro-choice would believe that he has not killed a human being and should be punished less than a baby killer. But those who are pro-life would say he's is a baby killer.