The rationale for vitamin K is as follows:

Newborns are born with reduced levels of all vitamin K dependent clotting
factors (II, VII, IX, X).  This is probably related to poor placental
transport of vitamin K and lack of intestinal colonization by vitamin k
producing bacteria at birth.  Vitamin K dependent coagulation factors
rise 3 to 5 days post birth, then decline some, and this decline seems
more marked in breastfeeding babies, babes with perinatal asphyxia, or
babes born to moms who have been on anticoagualants related to coumarin.

Hemmorrhagic disease of the newborn manifests thru bleeding in the GI
tract, oozing from puncture wounds, generalized bruising, oozing from the
umbilical or circ site, etc. It can occur 2 to 3 days post birth or even
earlier if the mom was on meds such as anticonvulsants, warfarin, or
antibiotics, but also can occur late in infants who did not get vitamin K
at birth and are breastfed (due ot low levels of vitamin K in
breastmilk), or if the infant was ill or preterm and has been on
antibiotics ()prolonged) which reduce normal intestinal flora and compete
for vitamin K in the liver.  Vitamin K is given prophylactically as an IM
shot to prevent hemorrhagic disease and only needs to be given once.
Vitamin K can be given orally, but it appears that the dose may need to
be repeated at least once, a week or so after birth, to ensure
effectiveness.

An interesting quote from Blackburn and Loper's (1992) Maternal, Fetal,
and Neonatal Physiology (p. 189):
        "The need for routine vit. K prophylaxis in all newborns has been
questioned.  Recent studies have suggested that althought the immunologic
and biologic activity of the vitamin K-dependent clotting factors is
reduced, the newborn appears to have normal levels of these factors and
is not vitamin K deficient.  In addition, concentrations of vitamin K-
dependent factors do not necessarily increase after vitamin K
prophylaxis, especially in health infants. Unfortunately, it is difficult
to identify which infants need this prophylaxis and which do not, and
clinical and reserch evidence clearly demonstrate the risk of hemorrhage
in some infants".

Hope this helps.

Ione Sims, CNM, IBCLC