When  an embryo is transferred, frequently the mother has been
given large amounts of supplemental estrogen to "prime "her uterus,
so this may decrease her milk supply.  In "natural cycle" embryo transfer,
there is no supplement  and I'd bet there wuld be no effect either way.
Also, sometimes large progesterone doses are used in the second
half of the cycle, but that ought to have less effect.  This subject
comes up fairly frequently in my GYN practice.
Lisa G. Matthews, M.D. FACOG Stamford, CT