There is no evidence that pierced nipple can cause negative impact on breastfeeding mothers. If you are trying to have nipple piercing while breastfeeding, you might consider waiting for your baby to stop nursing because it will only cause discomfort to the both of you. Also no piercer will will work on a pregnant and lactating mothers.
To have your nipple’s pierced or not, you and your baby can still enjoy the benefits of nursing as long as you are healthy. The milk production won’t be affected. It is best to remove the jewelry before nursing to avoid choking and other problem that may harm your baby.