Many mothers have a good milk supply but not what one would call an abundant milk supply. In that case it is very possible that the baby will not gain weight adequately with a nipple shield. Furthermore, as mentioned above, when a baby feeds through a nipple shield, the milk supply can even decrease (see the information sheet Slow Weight Gain Following Early Good Weight Gain). Even worse, if the milk supply decreases, it becomes more difficult to get the baby to take the breast without using a nipple shield.
Even if some justification can be found for using a nipple shield, starting one before the "milk comes" in is, in our opinion, not best practice. So many babies who do not latch on in the first few days, will latch on without trouble, even easily, when the mother’s milk "comes in," especially if the mother gets good help. If the mother believes that the nipple shield has dealt with her problem, she may not get help until it is too late. Here is just one email (identifying information deleted) of hundreds we could have included:
"My baby was born on xxx weighing 2.5 kg (5lb 8oz). I started using a breast shield when the baby was a few days old because my baby would not latch on; everything seemed to go okay, but somewhere around 3 weeks I began to notice she didn't seem to be sucking properly and by her one month check up she'd only gained an ounce."
So what now? After a month feeding on the nipple shield, it may be extremely difficult to get the baby to take the breast directly especially if the slow weight gain was due to the milk supply decreasing rather than the baby not getting milk well because of the nipple shield (both are, in fact, possible). The mother may have been asked to supplement. The mother needed a lot of support.
We believe it is better that a mother express her milk and give it to the baby by cup (or, if absolutely necessary, by bottle) rather than use a nipple shield. At least expressing milk will usually maintain the milk supply. See the information sheets When Baby Does Not Yet Latch, Finger and Cup Feeding and Expressing Breastmilk.
Using a nipple shield for sore nipples has the same problems as using it for a baby who will not latch on. Milk supply may decrease and the baby may not want to take the breast directly again. Furthermore, a nipple shield is not a good way to treat sore nipples, oftentimes it will make the problem worse and cause more trauma. True, I have heard from some mothers that using the nipple shield helped them get past the pain and they were able to get the baby to take the breast again without pain; this is not always the case and there are better ways of dealing with sore nipples (prevention being the best of all). See the information sheets When Latching, Sore Nipples, The All Purpose Nipple Ointment, Candida Protocol as well as the video clips.
If the baby is not restricted to starting breastfeeding at 34 weeks gestation (as in most of special care units or neonatal intensive care units in North America and Western Europe), if the mother is helped get the best latch possible and shown how to know a baby is getting milk, then nipple shields will hardly ever be necessary for the premature baby. See the information sheet Premature Baby.
A baby learns to suck and suck well by breastfeeding. If a baby "sucks better" on a nipple shield it's only because the baby is not latching on to the breast. A baby who latches on and gets milk will suck just fine. The problem is that the baby is not latching on well and using a nipple shield does not teach a baby now to do that.