Ongoing, long-term milk production depends mostly on milk removal by nursing or pumping. The more often milk is removed, and the more completely it is removed, the more milk the breasts make. The opposite is also true. When milk is removed less often, or an insufficient amount is removed, the breasts get the signal to slow milk production and make less. Milk removal occurs when a baby effectively breastfeeds or you are pumping frequently.
Breastfeeding requires effective sucking by the baby so that enough milk is transferred from the breast into the baby's mouth where it is swallowed. To suck, a baby must latch deeply onto the breast and use the structures in his or her mouth to create intermittent suction and squeeze milk into his or her mouth. Proper sucking signals the mother's body to release the hormone oxytocin. With the release of oxytocin, your milk will "let down." This is the term used to describe the flow of milk from the breasts.
So if your baby is premature or sick and unable to remove milk effectively enough to stimulate and maintain your milk production, you will need to express milk until he or she is stronger.