The unborn is not part of the mother's body, but rather a separate body that lies within her. Scientifically, there are a number of qualities and characteristics that are different about the unborn and the mother.
The unborn’s gender can be different than that of the mother. Furthermore, the unborn’s DNA is completely different from the mother. The unborn has a different brain and central nervous system, heart, heartbeat, blood type, and circulatory system than the mother.
The unborn’s blood and the mother’s blood come very close together, but they never mix. The mother’s body mounts an immune response against the unborn if there is a problem in the placenta because it knows that the unborn entity is a separate being unique from the mother’s body.
You can also conceive the unborn outside of the mother’s body through in vitro fertilization and later place that unborn child into the mother’s body. This is further evidence in that we’re not talking about the same body.
Finally, through surrogacy, you can have an unborn child that is a completely different race and ethnicity from the mother.
The claim that a mother can have an abortion merely because the unborn is part of her body is simply not scientifically accurate.