Breast is best, but it may not be easy.
Strike 1 - born too soon. It's tough for the mother of a preemie to get started with breastfeeding. Depending on just how early the baby is, it may be impossible to put him/her to breast for many weeks. The only stimulation she gets, unless she's tandem nursing a sibling, is from the pump. I've pumped for my full-term babies. It's not fun. It's frustrating. Sometimes it hurts. In the beginning you get little or no rewards for your efforts -- rewards being the liquid gold of breastmilk.
Strike 2 - Mom is sick. Often babies are delivered prematurely because there is a maternal problem - an infection, elevated blood pressure, placental abruption or placenta previa. Some of these complications require major surgery for delivery. The surgery and any blood loss can delay lactation. A mom who is being treated for pre-eclampsia may be unable to pump at all, much less regularly.
Strike 3 - Unrealistic expectations can lead to ineffective or infrequent pumping. Most new moms seem to think that they will have milk from the first time they pump. This just isn't realistic. Colostrum is present in small quantities from before the baby is born, but extracting it with a pump instead of a healthy newborn is sometimes an exercise in futility for the first day or 3. Our lactation consultants and the NICU staff know this and make sure that our moms know it as well. Getting them to actually hear what we're saying is a major challenge, though.
Frequent pumping, adequate hydration, and a good pump (preferably hospital grade) can all make a difference. One intervention that can help both mom and baby is skin-to-skin contact. Mom comes dressed in clothing that is easily rearranged to allow the diaper clad baby to snuggle against the skin of her chest. Even very small babies tolerate this well if their condition is otherwise moderately stable. I've seen very rapid increases in milk volume with a good pump and skin-to-skin time. Babies clearly enjoy this as well. They usually snuggle in and often go to sleep. If they don't sleep while being held, they will frequently go into a deep sleep when they are returned to their bed.
Skin-to-skin isn't just for preemies. It's one of the most effective interventions for getting term babies to focus and nurse as well.