Well...
looking at these statistics I'd say the largest reduction in infant mortality has been the increased care shown to Black mothers.
I'm sure sonograms represent 1 or 2 percentage points to those numbers across the board.
On an aside, it's sort of depressing that since 1995 there doesn't seem to be much progress in the reduction of that number. Certainly not as much as one would expect were progress advancing at a steady rate.
Perhaps it is the universally shortened maternity stay and casual treatment of mothers that has created the stagnation.