Kamari Yong receives breast milk at a Washington hospital. (Astrid Riecken for The Washington Post)

The weekly shipment arrived at noon Thursday — 300 ounces of breast milk donated by women across the country and pasteurized at a milk bank in Austin. It was packed with dry ice and shipped via FedEx to feed the most medically fragile premature infants in the neonatal intensive care unit at Children’s National Health System.

"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.

The percentage of advanced neonatal care hospitals across the country that provide donated breast milk has nearly doubled, from 22 percent in 2011 to nearly 40 percent in 2015, according to an unpublished analysis of data from the Centers for Disease Control and Prevention by Maryanne Tigchelaar Perrin, an assistant professor of nutrition at the University of North Carolina at Greensboro.

The rate was even higher — between 65 and 75 percent — for Level 3 and 4 NICUs that serve the smallest and most fragile premature babies.

The bulk of the donated milk is being provided by a network of nonprofit milk banks that are accredited by the Human Milk Banking Association of North America. The organization was established in 1985 to set safety standards for the industry, after nearly all of the 50 or so existing informally organized milk banks at that time shut down in the wake of the AIDS crisis and concerns about how the virus could be spread via bodily fluids.

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.

(The academy advises against informal milk sharing or using Internet-based sites to share milk, an increasingly common practice among parents of infants more broadly, because of risks of bacterial or viral contamination, or exposure to drugs or other substances).

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.

The NICU at Children’s started to use donated human milk in 2014.

The pasteurized milk goes to babies such as Kamari Yong, who was born on July 7 at about 23 weeks gestation weighing little more than a pound. Every day, nutritionists say, his mother expresses her own breast milk using a hospital-grade pump in her baby’s room, an arduous process that often results in just a few drops of milk.

So nutritionists at the hospital supplement with donated milk, putting the mixture into the feeding tube he uses to eat. Nine weeks after he was born, his weight has tripled and he is getting stronger. Nutritionists said that within a few weeks, they plan to switch him to formula.

Because of the relatively high cost and limited supply, donated breast milk is reserved for the smallest and most medically fragile babies, for whom research shows the most positive benefits from human milk. At Children’s, infants weighing more than 3.3 pounds are still given formula to supplement their mother’s milk.

Children’s pays about $4.75 per ounce for its shipment from the Austin milk bank. Formula, which is purchased in large quantities by the pediatric hospital, costs a small fraction of that sum. Many premature babies drink less than an ounce of milk every few hours.

The higher cost is a key reason that the use of donated breast milk is not universal. Some health advocates are concerned that hospitals serving primarily poor families are less likely to use donated milk. A survey of Level 3 NICUs published in 2013 found that “safety net” hospitals, where more than 75 percent of patients use Medicaid, were less likely than other hospitals to provide donor milk.

To prevent this economic divide in access to human milk, a handful of states including New York and Texas have approved measures to have Medicaid cover the cost of donor breast milk for premature infants. Lawmakers in those states have argued that the cost savings of treating a critical illness such as NEC far outweigh the price of human milk.

Some questions remain about the costs and benefits of human milk and cow’s-milk-based formulas.

One question researchers are studying is what impact cow’s-milk-based fortifiers have on the health outcomes of premature infants. Human milk, whether it’s donated or from the infant’s mother, is not considered nutritionally dense enough to meet all the needs of a preterm infant who, if still in utero, would be receiving food through the placenta.

NICUs typically add a fortifier to all the breast milk, whether it is from the mother or from a milk bank, to provide more protein and calories. One company produces a human-milk-based fortifier, but the cost is very high and not many NICUs use the product.

“We are able to help smaller and smaller babies survive and thrive, but what they need in terms of nutrition and protection . . . these are still cutting-edge questions,” said Naomi Bar-Yam, president of the Human Milk Banking Association of North America.

The Austin milk bank started dispensing milk in 1999.

In the past seven years, the number of hospitals it supplies has grown from 45 to 130. Last year, the milk bank had more than 1,000 donors who were screened for healthy diet and lifestyle choices as well as diseases such as hepatitis and syphilis.

The mothers typically are women who have given birth in the past 12 months and are producing more milk than their babies need.

They are not paid for the milk, and that is a distinction of nonprofit milk banks that appealed to the staff at Children’s.

“These are mothers who want to help other families who are in need,” said Judith Campbell, a lactation consultant in the NICU at Children’s.