Women who get magnesium sulfate during labour may have hypotonic
infants, but that is because the baby gets it through the placenta and
before breastfeeding is undertaken.  If the mother gets the mag
sulfate only after the baby is born, it should not affect the baby.
In the discussion, the mother was getting it orally for bowel
preparation for a test.  In this case there should be no effect.

Jack Newman, MD, FRCPC