The rules for knitting for NICU
Yarns to use
All babies' skin is delicate but in the NICU that is true times a hundred-fold. The younger the baby the more thin, delicate, and vulnerable their skin. With the very tiniest littlies, there are places where their is skin is only a few cells deep, so only the softest, most gentle, non-allergenic fibers can be used, 100% wool is preferred.
All sizes of yarn can be used; babies aren't just about baby yarn. There are two purposes for hats in NICU: keeping body warmth in and keeping noise out. Premature babies are not ready for stimulation of any kind yet, whether it's light, sound, or touch. Their development has not had time to progress that far. That's why NICU's are always dark, quiet, and warm. Noise and light that seems tolerable to us can actually damage the fragile still-growing cilia in their ears and rods/cones of their eyes. Because premature babies also lack the body fat yet to maintaint heir own body heat, we must supplement them with heated environment in their isolettes and with hats. Baby yarn is great, but worsted weight and bulky yarns can be appropriate and helpful too, as well as ear flaps (without any long ties that could present a choking hazard).
Colours to use
Historically, pastel colours have been used for babies, however babies are people too and come in all sizes, personalities, and skin colors. Pastels are great in the NICU but deeper, richer colors and variegated yarns are just as wonderful too.
The only colour we might caution against is yellow for micro-preemies (23-26 weeks). Because their organs are not fully developed, and skin is the biggest organ of the body, our littlest Littles' skin can be a little dusky at birth and the colour yellow isn't flattering. However yellow is perfectly fine for older preemies.
One of the great things about embellishments on hats for preemies vs toddlers, is that preemies don't chew on buttons, so within reason, as long as all embellishments are triple super duper sewed on tightly, they can be used on NICU hats. Try to keep anything sewn on to the front of the hat and away from the sides and back of the head so the baby won't be laying on any kind of applique, silk flower, etc. And the same goes for bulky seams. Try to stay away from bulky seams that the baby might lay on. Again, it goes back to their tender skin.
Thank to Glenna Muse for this great information! Check out her blog here.