If the cell is the smallest functional unit of life, then the smallest functional unit of human existence would be at the moment of fertilization. From that point on, the life form is biologically human and independent of its parental donors of genetic material and it does not meet the criteria to be classified symbiotic or a parasite.
Medically speaking, there is no question of its humanity--the notion of personhood is besides the point. Whatever criteria of personhood you believe an early-stage human lacks, there are individuals lacking those same traits well after birth.
For these reasons, I personally am Pro-Life: I believe that all human life has value and my sig sums up the reason why. However, when it comes to legislation/the Law, I am Pro-Choice. I do not believe that my values should be enforced upon the general population. I understand that my position may be morally praiseworthy, but not morally obligatory, and there are plenty of practical reasons to offer all women the option to decide for themselves where they stand on this issue.
