It makes sense to me for a physician to tell someone to continue their
     diet plan post-birth, but I am unclear how the infant would become
     hypoglycemic from the mother's diet while breastfeeding.  Infants born
     to diabetic mothers often are hypoglycemic at birth and this needs to
     be monitored and treated as needed. Unless the infant is extremely
     hypoglycemic, the best advice is early and frequent feedings of
     breastmilk. But the mother's diet is useful to maintain her own stable
     glucose level. Hypoglycemia for a new mother is common during the
     early postpartum period, especially if she maintaining tight control
     of her glucose levels. Hope this helps!