There is no evidence that taking prenatal vitamins decreases milk
supply.  There is some evidence that *large* amounts of B6 *during*
lactation will decrease milk supply.  The amount in prenatal vitamins
will not likely do it.  The mother does not need prenatal vitamins
after the baby is born, but if she's paid for them and has some left
over, she might as well finish them.

Jack Newman, MD, FRCPC