From Carl Sagan

Very interesting thoughts from Carl Sagan:

What was the reasoning in Roe v. Wade? There was no legal weight given to what happens to the children once they are born, or to the family. Instead, a woman's right to reproductive freedom is protected, the court ruled, by constitutional guarantees of privacy. But that right is not unqualified. The woman's guarantee of privacy and the fetus's right to life must be weighed--and when the court did the weighing' priority was given to privacy in the first trimester and to life in the third. The transition was decided not from any of the considerations we have been dealing with so far…--not when "ensoulment" occurs, not when the fetus takes on sufficient human characteristics to be protected by laws against murder. Instead, the criterion adopted was whether the fetus could live outside the mother. This is called "viability" and depends in part on the ability to breathe. The lungs are simply not developed, and the fetus cannot breathe--no matter how advanced an artificial lung it might be placed in—until about the 24th week, near the start of the sixth month.

This is why Roe v. Wade permits the states to prohibit abortions in the last trimester. It's a very pragmatic criterion

http://www.2think.org/science_abortion.shtml