In addition to specific diagnoses, infants in the NICU can experience other general problems. For instance, newborns lose heat easily, and premature infants in particular have trouble regulating their body temperature, as they lack the energy or fat reserves to generate heat and the body mass to maintain it. So NICU babies must be kept warm in warmers or isolettes. High or low blood pressure can also be a risk for premature babies because their developing blood vessels can't handle changes in blood pressure and may tear more easily.
Some premature infants have trouble feeding because they are not physically coordinated enough yet to do it. Eating is the most energy-consuming process for a newborn, and babies in the NICU often do not have the strength or energy to feed on their own. Instead, they have to be fed through an IV line or a tube. And if the digestive tract is not sufficiently developed to handle food, then that can cause problems as well, as seen with NEC.
A related condition is reflux. Although all infants have some reflux in the early months (hence all the spitting up), premature babies have a particular problem with it because they have poor muscle tone. Sphincters are muscles, and when the one between the esophagus and the stomach is weak, it allows the acidic stomach contents to bubble back up into the esophagus. (A preemie's immature nervous system has trouble controlling the sphincter as well.) The acid irritates the esophagus, which can lead to feeding problems. Inhaling and choking on the reflux is a more serious risk.
Newborns who need intensive care are also vulnerable to infections. Their skin and mucus membranes - the body's primary lines of defense against bacteria and viruses - are not well developed enough to provide adequate protection. With several immune-compromised babies in one space, infections that are introduced into the NICU can spread easily, and with nurses caring for multiple babies, the possibility of communicating infectious agents increases. That is why NICU staff are vigilant about keeping that environment as clean as possible.