The right of life, which is the primary, necessitates logically that the entity be alive. A dog or a cat, for example, do not have the mental capacities to understand and act on their hypothetical rights. That is why we consider them to be domestic animals, insofar as we deduce that they are alive but do not have mental capacities far beyond that point. Some people would argue that rights only apply to humans, since ethical deductions are human in nature, and that is an interesting topic of debate, but outside of this discussion. Human fetuses, on the other hand, are future human adults.
Nobody can deny that normal human adults have full use of their rights.
It is a part of her body, much as her heart is. The government does not own a woman's uterus any more than it owns any other part of her body.