The slogan "human life begins at conception" embodies pro-life advocates' position that a new human being comes into existence at the moment an ovum is fertilized. The unstated assumption behind the slogan is that "personhood" begins at the same time as "human life", and therefore the fertilized egg is a person with a right to life from the very moment of conception.
Pro-choice advocates argue that the slogan human life begins at conception is potentially confusing or misleading. They agree that it may be technically true in one sense, but charge that it deliberately uses ambiguous language to gloss over the crucial issue of the time personhood begins at - an issue on which there is a wide scale of opinion. They attribute any force the slogan has to the exploitation of this confusion.
Pro-choice advocates, while acknowledging that the living biological tissue of an early fetus is human - just as body parts such as a white blood cell, sperm cell, or a finger are "human" - do not consider the early fetus to be a person. Most do concede, however, that as the fetus develops and approaches birth, its claim to be considered a person increases.
A moderate pro-choice position is that personhood begins at viability; an extreme position is that it begins at birth. Other positions exist.
Because the fetus is in a constant state of development, many on the pro-choice side would admit that it is difficult to find a sharp cut-off point at which it obviously becomes a person. This is one reason why the "at conception" slogan has the appeal (to some) that it does, as it seems to offer a clear, unambiguous solution.
Those who posit a fertilized egg is "a person" at conception are also implying that "one person" can in fact split into two or more persons: in the case of identical twins (or triplets, etc) the "fertilized-egg-person" of one cell becomes two or more "persons" before birth. This is considered by some to be a reductio ad absurdum of this position, though some theories of personal identity can accommodate this "fission".
See also: abortion, legal and moral issues, religion and abortion, Slogan:A woman's right to choose
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