It can't be enforced without massive government involvement--or with the help of "concerned citizens" (rats, snitches, etc--people that turn in women they suspect of having had an abortion).
Which is why I think that such a law or amendment would be in the books more as a "feel good" law, just to make the pro-life stance official. If they can catch a few women or doctors here and there, they'd be happy.
Also, it's worth pointing out that the rest of the amendments (except the boring 27th amendment) are about having government "back off" from the citizens, granting them more freedom. This amendment would actually be a step in the other direction, adding restrictions on our freedoms (as would any marriage amendment).