I have been away for a few days, and I apologise if this has already been
addressed.
Iron in vitamins or formula does NOT cause constipation.  There are several
good research studies to show this.  Premies sometimes have slow GI motility,
and in order to increase feeds many Neonatologists will use Reglan, Cisapride
and/or glycerin suppositories.  It is usually the artificially-fed infants who
have a problem, not the breastmilk fed, but if fortifier is added, the stools
look, and act, more like the stools of an artificially-fed infant.  Many
premies have infrequent stools at first when they get home - true cinstipation
is rare.  Glycerin suppositories are over the counter and can be tapered
slowly if an infant is believed to be "dependent" on them.
Nancy Wight MD, FAAP, IBCLC
Neonatologist