"None of the Justices believed that the right to abortion extended to the very moment of birth. At some stage in the pregnancy, the state’s interest in protecting the fetus became sufficiently compelling that states could proscribe abortion in almost all cases other than when necessary to preserve the woman’s life or health. To determine when that point occurred, Blackmun effectively had to decide when the life of the fetus “began” at least to the extent of deciding when the state’s interest in protecting the fetus became compelling."