State health commissioners announced on Tuesday that letters highlighting the importance of breastfeeding were being sent to hospitals, reminding them of regulations limiting unnecessary formula feedings for breastfed newborns.

The state initiative coincides with Bloomberg’s call for hospitals to lock away their baby formula and have nurses encourage new mothers to breastfeed.

Under the mayor’s plan, slated to start September 3, the city will keep a record of the number of bottles that hospitals stock and use. Formula would be signed out like medication.

The pro-breastfeeding campaign has drawn the ire of some women who argue it stigmatizes infant formula and interferes with a mother’s choice of what to feed her child.

A number of the city’s other health initiatives — including cracking down on large-sized sodas and banning smoking in public places — have attracted similar criticism from those who accuse the mayor of creating a “nanny” state.

It’s not clear how many women will take adavantage of the new policy, but the US Health and Human Services Department estimates that 47 million women, ages 15 to 64, have private health insurance plans that will be affected. The 2010 health reform law requires policies provided by private health insurance companies pay for a list of women’s health preventive services, starting August 1.

However, there may be a delay in services for many women. The law applies to new policies — women with existing coverage may have to wait for their policies to renew for the requirements to kick in, which could take months. Many health insurers already provide this coverage.

The new rules are based on guidelines from the independent, non-partisan Institute of Medicine, which said paying for these services will save money and lives down the road.

“We want healthy women to have healthy babies,” said Dr. Jennifer Howse, president of the March of Dimes Foundation, a charity that works to prevent birth defects. “Receiving regular medical care greatly increases the likelihood that important messages can be delivered to pregnant women around issues such as nutrition and tobacco cessation, and provides opportunities to detect potentially dangerous conditions like gestational diabetes or high blood pressure.”

There are a few exceptions. Purely religious employers don’t have to provide the services to employees if they object. Related groups, such as Catholic-affiliated universities, have objected so the Obama administration offered what it called an accommodation, forcing the insurance companies themselves to pay for the coverage. But the religious associations still object, as do Republicans in Congress. They have promised to repeal the whole law if they win enough seats in the November election.

Researchers found that women who had children tended to have higher body mass indexes later in life than did women with no children; however, the researchers were able to associate every six months of breast-feeding  with a 0.22 drop in BMIs among the women in their 50s and early 60s.

This translates to a 1 percent drop in BMIs for every six months of breast-feeding, the researchers said.

“We already know breast-feeding is best for babies, and this study adds to a growing body of evidence that the benefits extend to the mother as well, even 30 years after she’s given birth,” said study researcher Dr. Kirsty Bobrow, a researcher at the University of Oxford.

“We applaud the effort of all of the hospitals to make this explicit statement of their support of breast-feeding here in the Commonwealth,” said Dr. Lauren Smith, the public health department’s medical director.

Back in 2005, Massachusetts tried to end the free formula practice with a statewide ban instituted by the Public Health Council, but that decision was overturned several months later when then-Governor Mitt Romney replaced council members who were in favor of the ban.

More than a dozen studies have shown that breast-feeding mothers who received free formula samples after they left the hospital were less likely to be breast-feeding by the time their infant was one month old. “Using formula without a medical reason is one of the biggest predictors of breast-feeding failure,” said Dr. Melissa Bartick, chair of the Massachusetts Breastfeeding Coalition.

But infant formula makers responded that formula giveaways have been inappropriately blamed for women opting out of nursing because it’s, for example, to difficult to maintain when they head back to work. “Some critics of formula samples claim research has ‘consistently shown’ that samples in discharge kits negatively affect duration of breastfeeding,” the International Formula Council, an industry group, said in a statement. “In fact, the research results have not been consistent. Some studies show an effect, while others do not.”

“I didn’t set out to nurse a 3-year-old,” said Jessica Cary of Park Slope, whose daughter Olive continues to breastfeed. “But two years came and went. Now breastfeeding and mothering are so intertwined for me.”

Government agencies don’t track breastfeeding past 12 months, and many pediatricians assume it has stopped by the child’s first birthday. So there’s no way to know how many of the 28 percent of New York babies who breastfeed up to age 1 keep on going.

Long-term breastfeeding moms often cite the World Health Organization, which encourages nursing until at least age 2. Neither WHO nor the American Academy of Pediatrics sets an upper limit on breastfeeding’s duration

“Experienced pediatricians realize that the benefits of breastfeeding don’t just magically disappear after one year,” said Karen McGratty, a lactation consultant in Midwood who is nursing her 3-year-old son.

Most mothers of breastfeeding preschoolers let the child take the lead in weaning.

“At this point I don’t offer nursing, only give it to her when she requests it,” said Cary. That leads to a gradual reduction on the child’s own timetable.