Many factors, including the length of maternity leave, affect how long a woman breast-feeds.

According to a survey by the Centers for Disease Control and Prevention, about 75 percent of the 4.3 million mothers who gave birth in 2007 started breast-feeding. By the time the baby was 6 months old, the portion dropped to 43 percent, and on the child’s first birthday, to 22 percent.

A study released this year by Harvard Medical School concluded that if 90 percent of mothers followed the standard medical advice of feeding infants only breast milk for their first six months, the United States could save $13 billion a year in health care costs and prevent the premature deaths of 900 infants each year from respiratory illness and other infections.

“The old adage that breast-feeding is a child’s first immunization really is true,” said Dr. Robert W. Block, president-elect of the American Academy of Pediatrics. “So we need to do everything we can to remove the barriers that make it difficult.”

To continue breast-feeding once they return to work, many mothers need to use pumps to extract milk, which can be chilled and bottle-fed to the child later. The cost of buying or renting a breast pump and the various accessories needed to store milk runs about $500 to $1,000 for most mothers over the course of a year, according to the United States Breastfeeding Committee, a nonprofit advocacy group. Lactation consultants, who can cost several hundred dollars, also would not be an eligible expense.

Roy Ramthun, a former Treasury Department official, said that tax officials’ reluctance to classify those costs as medical expenses stemmed from a fear that the program might be abused.

“They get very uneasy about anything that smacks of food because they fear it will open up all sorts of exceptions,” said Mr. Ramthun, who runs a consulting company that specializes in health savings accounts. “It’s a matter of cost and of protecting the integrity of the tax code.”

Bills introduced last year by Representative Carolyn B. Maloney, Democrat of New York, and Senator Jeff Merkley, Democrat of Oregon, would have allowed nursing mothers to claim the tax break. But breast-feeding advocates say that effort, like many before, was undone by economic and cultural factors.