Infant mortality is a well established term for referring to the left-most (decreasing) portion of the even-more-well-known Bathtub Failure Rate model.
Here are some citations from google book search:
“Dynamic Business Strategy: The Art of Planning for Success”
page 19.2
1977
“Advances in Instrumentation”
v.41 pt.1
page 144
1973
“IEEE Transactions on Electron Devices”
v.26 1979 Jan-Jun
page 38
orig. pub 1963
See also this page:
URL
which describes the bathtub model in detail and cites a study in the 1950s as originating it. It also cites a work (of which I would only find newer copies online) published in 1961.