Pros: sometimes may help; much better than the old thicker "rubbery" ones
Cons: may decrease milk supply, leading to inadequate intake for baby and early weaning
These nipple shields *can* save breastfeeding, but if you think you need a nipple shield at all, *first* you need to consult a Lactation Consultant. Why? Because nipple shields (any brand) are definitely a mixed blessing - you can do more harm than good with them, and you may still not find the solution you are looking for.
When are nipple shields recommended?:
Sometimes a mother's nipple doesn't stick out far enough, or isn't firm enough, for the baby to get a good latch on the breast. A baby that's nursing well doesn't actually grasp the nipple itself, but the breast tissue beyond the nipple; this is where the milk-making apparatus is and this is what the baby needs to compress in order to 1) get milk to flow and 2) stimulate the breast to continue to produce milk. The nipple is really just a spout (picture a teapot), and it serves as a bulls-eye to the baby who is searching for food - "try here, right here, the dark circle marks the spot, kiddo, here's where the goodies come from". Sometimes if the breast is so engorged that it is hard (picture a soccer ball), the baby can't compress it enough to get the whole works in his mouth, and if this is happening a nipple shield might help. (Temporarily, while you work on getting rid of the engorgement!)
Other women may have nipples that are long & soft, or unusually short, or don't protrude enough for the baby to get a mouthful of breast. In some cases, it may not be an anatomical problem with the breast, but rather that the baby is not taking the breast well (for any of a number of reasons). Nipple shields may be of some use in these situations, but they are not to be used casually without the guidance of a professional.
Occasionally, use of a shield may help a very sore nipple to heal. I don't recommend them for this purpose, though, as the suction & friction actually may aggravate nipple pain. The cause of the pain needs to be addressed in order for it to truly heal.
So what's the problem?:
1) Nipple shields, even the "best" ones (which in my experience as an IBCLC the Medela shields are) may cut down on milk production. Milk supply (production) is driven totally by demand (milk removal by either the baby or a pump). When the baby latches and sucks correctly, the breast gets the appropriate stimulation to continue producing the required amount of milk. Because the shields are something of a barrier, there is less breast-to-baby's-mouth contact (which equals less stimulation), and this leads to a drop in supply. The mom feels the baby sucking, so doesn't worry about her supply, but it can be seriously decreased without her awareness, leading to more problems.
2) Babies (and mothers!) can get "hooked" on the shield, creating a form of what is commonly called "nipple confusion", although a better term for it is "suck confusion". The baby learns to associate the pleasures of food and nursing with the feel of the shield in his mouth, and may refuse to nurse without it. Over time, this will almost guarantee a decreased supply and lead to premature weaning of the baby. Besides that, it's a pain in the neck to be stuck needing a gadget to nurse - this is supposed to be *easy* and *natural*, right?
3) The shield itself can very easily harbor germs or (much more frequently) the yeast that causes "thrush" (an overgrowth of yeast, normally kept under control by the body's "good bacteria"). The organism responsible, Candida albicans, is very easily transferred back & forth between mother and baby, because it stays "alive" in the nipple shield, as it does on pacifiers, toys that go in the baby's mouth, and towels/washcloths used back & forth between an affected mother & baby. Frequently, thrush goes undiagnosed - and the only way your baby can tell you that his mouth is sore is by fussing & pulling off the breast, which may be part of what has led you to use a shield in the first place!
Whatever is causing a mother to believe she needs to use a nipple shield should lead her to consult a lactation consultant (preferably and IBCLC, or International Board Certified Lactation Consultant) for help in finding a good workable solution. A shield may be a legitimate part of that solution, but if your baby isn't latching on consistently and nursing steadily for several minutes at a time,at least 10-12 times a day, you need a little expert support and guidance - and that's what we LCs are here for!
(By the way, I do recommend this brand of shield far more highly than the others, if a shield is what's called for.)