The 14^th Amendment to the U.S. Constitution defines who is a citizen: “All persons born or naturalized in the United States and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside. No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States , nor shall any State deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws”

This amendment is pretty clear and reflects the fact that at the founding of our nation the right to abortion before quickening was an inalienable right as outlined by Blackstone.

http://press-pubs.uchicago.edu/founders/documents/amendIXs1.html

“No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States , nor shall any State deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws” Applies to the woman who is a citizen, not to the fetus that is not born.