Laura:  Although it's probably OK, I remember reading that doxycycline is
less bound to calcium than tetracycline.  That's why we can prescribe it
for adults to take with food, including milk, and they will absorb it.  If
a tetracycline is the only type of antibiotic possible for her problem
(which I doubt), then plain tetracycline is probably the best choice
because any of that which gets into her milk will be bound to the calcium
in her milk and not absorbed at all by the baby.  I know of no condition
requiring doxycycline which cannot be treated equally well by
tetracycline.  Hope this helps.  Alicia Dermer, MD, IBCLC.