From the mid-1800's until 1967, most, and then finally all the states of the US banned criminal abortion in their law codes. In no case was there ever a prosecution for natural miscarriage, even if the mother were arguably negligent (e.g. chronic drunkenness during pregnancy.)
You forget that mens rea -- the Latin term for "guilty mind" -- is usually one of the necessary elements of a crime. This is absent in any case where miscarriage results from anything except an abortive attempt.
Furthermore, law enforcement requires the willingness of a prosecutor to prosecute. No prosecutor --- and I mean none of them --- wants to investigate miscarriages. In the real world, it's simply an impossibility. And that evaluation is based on U.S. experience for over 100 years.
If the Catholic church said that “1 + 1 = 3” and a teacher was hired to teach “1 + 1 = 2”, then he must teach that. I have no problems with a man of God saying he doesn't use the Bible or the Catholic church's teaching to interpret cases."
This is not a situation where the Catholic Church or the Bible are needed to establish by supernatural revelation that child is a human being, whether that child has been born or is in the womb traveling towards birth. It is not a matter of "revelation" or "religious authority" because the fact is well-attested-to by the rapidly developing sciences of embryology and perinatology.
It is not unreasonable to demand that the law conform to the advances of scientific knowledge.
And you forget that involuntary manslaughter can consist merely of “recklessness.” There need be no intent to hurt another for this to be found. And simply because this has not been found to date for this situation, it does not mean that the law can’t be interpreted to make this so, going forward.
http://en.wikipedia.org/wiki/Manslaughter
It wasn’t until the past century that abortion was “found” in our laws to be legal. Without a constitutional amendment that finally disallows only abortion, then the courts are left up to interpreting what they want from other cases and laws.
We either want someone who will interpret as they did originally, meaning abortion was never allowed based on original intent, or a constitutional amendment that stopped this new practice, in the absence of the Supreme Court overturning it based on some case.
As for prosecutors not wanting to go after a crime, they are required to do so, even if they don’t want to.