A growing body of research shows that human milk carries long-term benefits for premature infants. Picture: Pexels

"Liquid gold," said Victoria Catalano, a NICU dietitian at the children's hospital in Washington, holding up a plastic bottle containing three ounces of frozen milk. Then she corrected herself. "Well, that's liquid gold," she said, pointing to two large deep freezers stocked with milk the infants' mothers had produced. "This is the next best thing," she said.

A growing body of research shows that human milk carries long-term benefits for premature infants and can be lifesaving, but it's often hard for mothers of premature infants to produce enough. Historically NICUs have supplemented feedings with formula, but now they are increasingly looking to milk sharing - a practice with roots in an ancient tradition of wet nursing - as the nutritional vanguard for babies who are born too soon.

Today there are 23 accredited milk banks in the United States and three in Canada, twice the number five years ago. A handful of private milk banks have also sprung up to meet the growing demand for human milk in NICUs.

The American Academy of Pediatrics in 2012 recommended that donated breast milk rather than formula be used for preterm infants when a mother's milk is unavailable. The policy statement cited a range of benefits, including the prevention of sepsis and other infections as well as long-term benefits in growth and brain development.

Studies have shown that human milk can also protect against necrotizing enterocolitis, a disease that causes serious damage to babies' intestines. NEC, which affects 12 percent of babies born weighing less than 3.3 pounds, is one of the leading causes of infant mortality in the United States.

Donated breast milk in general is less nutritious than milk produced by an infant's mother, which changes in composition - with varying levels of proteins, fatty acids and white blood cells - to meet the unique immunity and nutritional needs of her own baby. But for many reasons, it's not practical or possible for many mothers to provide milk, particularly for premature infants.

Just as their babies did not have time to mature in utero, often the mothers' bodies were not prepared for birth, neonatologists say. The stress of being in a NICU also makes it hard to produce milk, said Kim Updegrove, executive director of the Mothers' Milk Bank at Austin, which is one of the largest in the country.

"These babies are in incubators with beeping equipment and tubes coming from them and going into them. It's not the picture of parenthood that anyone expected, and that works against our ability to provide milk to human babies," she said.