Kneading is what you do to bread dough - it's done with the paws.
Suckling is the mouth action part of nursing behavior.  Both are part of
nursing behavior: the kitten kneads his mother to stimulate milk flow
while suckling actually gets the milk.  Mammals are programmed to find
these actions pleasurable so that they will do them as often as possible
and grow as fast as possible.  When they grow up and are weaned, the
pleasure of the actions doesn't go away, the mother just refuses to allow
herself to be the object of them.  While actual suckling is likely to
disappear with time because it no longer results in milk, kneading may
continue because it is a pleasure in itself.  This is obvious from the
purring and liik of satisfaction seen in adult cats when they are kneading
a person, another cat, or the bed sheets.

That happens, and it costs more to have them neutered when pregnant - if
the vet will do it and the pregnancy is not too advanced.

I have fifteen cats (all neutered) at the moment, and have been living
with cats for about thirty years - I have seen just about every behavior
that cats are capable of, including a couple of unexpected pregnancies.