Fiona, there are several thoughts which popped into my brain after reading
your post about the seemingly well 7 week old who suddenly has poor
oral-motor function.  The first was botulism.  I had a mother who brought in
an infant who was recovering from botulism, and she described an early
presentation of symptoms similar to what you describe.  She had baby in to
see doctor several times:  baby never had fever or other overt sx.  Just
floppier and floppier.  She finally insisted baby be in hospt. for tests as
feedings got less and less effective.  Baby stopped breathing in hospt.
parking garage.  Mom ran for ER and luckily story had happy ending.

The second, and less dramatic thought I had was that sucking is a complex
neurological process (duh!).  Sometimes maturational issues produce
temporary dysynchronies.  In some babies this is observed around 6 months
when the skull elongates.  However, I don't think it is the norm in a 7 week
old.  I always recall my personal animal model when dealing with babies:  If
the dog stops eating, I say to my husband, Damn.  Now I have to take him to
see the vet!  We recognize that failure to eat, or reduced ability to eat is
a sign of illness in animals. In my mind, it is in humans as well.
Barbara

Barbara Wilson-Clay, BS, IBCLC
Private Practice, Austin, Texas
Owner, Lactnews On-Line Conference Page
http://moontower.com/bwc/lactnews.html