Rebecca Soltes walked into a gaily decorated hospital meeting room, pushing her kids in a double stroller and carrying a soft-sided cooler filled with bags of frozen breast milk — 100 ounces of the stuff.

She dropped it on a table, and the roomful of lactation consultants and maternal health advocates from California's Inland Empire erupted in applause.

She was donating the milk because she didn't need it anymore. Her son had recently graduated to solid food, but her freezer at home was still packed with pumped milk.

"I didn't know what to do with it," a harried-looking Soltes told two staffers from the Mothers' Milk Bank, a nonprofit in San Jose, Calif., that provides breast milk for fragile infants in neonatal intensive care units.

Increasing numbers of women who produce more breast milk than they need are handing it over — or selling it. It's a boon to fragile infants and mothers who can't produce enough milk, but it also poses challenging ethical and public health questions.

The Food and Drug Administration does not require testing for donated human milk.

A handful of states, including California and New York, regulate milk banks the way they do tissue banks, enforcing some safety standards, and many nonprofit milk banks screen donors.

Minnesota does not regulate breast milk donation, said state Health Department spokesman Doug Schultz.

But the FDA has expressed concern about milk-trading websites, warning consumers that the milk offered there carries an increased risk of contamination by drugs or disease, including HIV. The agency further urges mothers not to feed their infants donated milk acquired from other individuals or over the internet, also because of such safety risks.

Some companies and medical experts wonder if even established milk banks — whose voluntary screening practices the FDA praised in 2010 — should be doing more to ensure safety. Dr. Jae Hong Kim, a neonatologist at the University of California, San Diego, said milk banks' safety efforts have worked well thus far but are not error-free. "The challenge is, if you expand the operations, you increase your risk exposure," he said.

Scott Elster, CEO of Prolacta Bioscience, a company that uses human milk to make fortifier products in its pharmaceutical-grade manufacturing facility, said he advocates a more rigorous level of screening.

"We believe that all human milk, whether it is distributed commercially, by a nonprofit or peer-to-peer network, should be tested for drugs of abuse, nicotine and other adulterants," he said.

As recently as the early 1970s, barely a fifth of American babies were breast-fed. Today, more than 80 percent get at least a taste of their mother's milk, according to the federal Centers for Disease Control and Prevention. The health advantages are well-established, especially for premature infants.

"The first class I had on human milk I was blown away," said Maryanne Perrin, a nutrition scientist at the University of North Carolina-Greensboro. "It's a magical food."

No one knows exactly how much excess milk is donated or sold, but the volume is increasing. Pauline Sakamoto, executive director of the Mothers' Milk Bank and former president of the Human Milk Banking Association of North America, an association of nonprofit milk banks in the U.S. and Canada, said her organization's affiliated banks distributed about 4.4 million ounces of human milk to hospitals last year, up from less than half that amount as recently as five years ago.

For-profit breast milk operations are expanding, too. Elster's company, Prolacta Bioscience, stocks warehouse-like freezers full of human milk.

Based in City of Industry, Calif., it purchases the milk from women for $1 per ounce. The company uses it to make fortifiers that are added to a mother's or donor's milk to deliver extra calories, proteins and minerals to premature babies.

The products are expensive, at $200 to $300 for a one-day supply. But they win praise from NICU physicians like Kim, because they can replace fortifiers made of cow's milk, which are known to increase the risk of necrotizing enterocolitis and sepsis.

Beyond the NICU and the gates of industry, informal internet-based sites also appear to be booming. One report estimated that the number of online milk transactions in the U.S. soared from 22,000 in 2012 to 55,000 in 2015.

Some milk-sharing networks are run by mothers who sell their milk, while others give it away.

Research shows there's some risk with milk that changes hands on informal sites, however. In 2015, a team of researchers who purchased breast milk on the open market reported in the journal Pediatrics that about 10 percent of samples contained cow's milk, which could have been added by the sellers to increase volume and thus the size of the payment. The same team had reported in 2013 that milk purchased online was frequently contaminated by pathogenic bacteria.

Kim Updegrove, executive director of the nonprofit Mothers' Milk Bank at Austin, in Texas, says the FDA should regulate informal milk-sharing, ideally requiring all milk to go through a milk bank focused on distributing it according to the greatest medical need.

The entire concept of breast milk banking — of any type — was new to Soltes. The La Quinta, Calif., mother said she learned about the milk drive by doing a Google search.

"I thought, wow — I'd rather do this than throw it all out," she said.