As Maureen Minchin pointed out, an organ will only respond to oxytocin
if receptors are present. Lawrence's book (1989 edition) mentions
that there is a sudden 40-fold increase in oxytocin receptors in the
uterus in the hours before delivery. I would imagine that the uterus
would be relatively insensitive to oxytocin before that time, unless
some other problem had predisposed the mother to miscarriage that
was totally unrelated to breastfeeding.
There has also been research (on women, not wallabies!!) to show
that there seems to be changes in both the sensitivity and number
of oxytocin receptors in the breast during pregnancy - eg the dose
of oxytocin needed to cause a change in intramammary pressure
decreased significantly during late pregnancy and lactation compared
with the non-pregnant state.  I wonder if the oxytocin receptors on
the uterus may also change in sensitivity to oxytocin just prior to
labour. This would seem to be a useful evolutionary step, but I am
wildly speculating here!!

Fiona Coombes  MBBS IBCLC
Perth Western Australia