Possible neurological effects to baby breastfed during mother's stomach-cancer treatment? (chemo etc.?) obvious mental retardation only or subtle too?

Difficult to answer: Many things make a difference here - what chemo is being used, how long exposure, etc. Remember, your baby is going through tons of development of every organ system and chemo will harm this. While i applaud your wanting to breast feed, i can't imagine risking your child's development, intelligence and risk for cancer in the future just to continue breast feeding. Effects could be devastating.