The Federal government has no business involving itself in matters that should instead be left to criminal statutes in individual states -- unless, of course, it involves matters specific to Federal law (crimes involving interstate commerce, for example) or involves a clarification of constitutional matters as they pertain to groups of people (the Civil Rights Act of 1964, for example).
The only legitimate Federal involvement in this case would have been a law that codifies full constitutional protection for the unborn child -- or a law that eliminates all constitutional protection for unborn children. There is no room for anything in between.
To get an idea of how useless and irrational this Unborn Victims of Violence Act is, imagine how preposterous it would seem if Congress had passed a Civil Rights Act in 1964 that only applied to people living in states east of the Rocky Mountains and west of the Mississippi River.