Hi,
Nipple shields can be a useful tool in the short term to help a baby attach to the breast. Usually flat (ish) nipples pose no problem for the baby. They "imprint" their mother's nipple - that's why ideally we hope for an unmedicated birth, early skin to skin and no interruption for mother and baby while they get their first feed underway somewhere within the first hour. I have heard widely differing professional opinions of "flat" and even "inverted". Sometimes, when breastfeeding does not go well, and baby can't get onto the breast, then nippleshields can be a very helpful little device. Research by Paula Meier even shows that prem and near term babies transfer more milk via a shield. The first rule is always "feed the baby" and if a nippleshield can help that feed to be at the breast, then that is preferable to any other alternatives (ie cup, syringe or bottlefeeding)
Is the nipple shield the right size for the mum? I have seen mums whose nipple (although flat to begin with) is drawn into the shield and actually presses out through the little holes in the shield (ouch) Is it centred over the nipple? Sometimes you feel like you need to be an octopus to hold the thing in place while feeding. Sometimes wetting the shield with milk or water helps it stick. Has the mother's milk come in? Many mothers find it painful before the milk comes in. If on the other hand, her milk is in, and she is very full, expressing a little milk by hand or with a hand pump before she attaches (with or without the shield) might be helpful. Any evidence of tongue tie?
Lots of skin to skin with mother and baby will enhance attachement. Have you seen the latest "Baby led attachement video clips? (some-one will pop in with the URL I'm sure)