Before her second son, Hayes, was born a year ago, Erin Carroll was determined to breast-feed him.

Courtesy Erin Carroll

Erin Carroll and second son, Hayes, seen here at 4 months. Erin gave up breast-feeding Hayes when a hospital nurse insisted he needed formula because he'd lost too much weight.

“With my first one, I didn’t really have any expectations or plans for anything,” says Carroll, 30, a stay-at-home mom in Columbia, S.C.

Breast-feeding didn’t exactly work out with her first son, Hudson, three years ago. Hudson weighed nearly 10 pounds at birth, and, Carroll says, she couldn’t produce enough milk for him. He lost nearly 2 pounds, and by the time he was 6 weeks, she was feeding him formula exclusively.

With Hayes, “it turned out that I was making enough for him, but he just fell asleep any time I tried to nurse him.” Before they left the hospital when he was 3 days old, Carroll says, a nurse told her “he’s lost too much weight. We’ve got to do formula.” Carroll says, “I was devastated.”

Carroll is like many women with the best intentions about breast-feeding, according to a new study from the Centers for Disease Control and Prevention, one of the first to examine the discrepancy between how long pregnant women say they intend to exclusively breast-feed and how long they actually do.

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Carroll continued supplementing Hayes’ feedings with formula and by her first postpartum doctor’s appointment four weeks after his birth had decided to stop nursing him altogether. 

“Omigosh, this formula is so easy,” Carroll recalls thinking. “It was sad at first, because I really just thought it would be different [with Hayes], and I didn’t have the same issues with both of them.”

Roughly half of the women in the new study said before they delivered they planned to breast-feed exclusively for at least three months, the CDC researchers report Monday in Pediatrics. But only a third of those women actually achieved their goal.

“The one that shocks me is the fact that 42 percent stopped in the first month,” lead author Cria Perrine, an epidemiologist in the CDC’s Division of Nutrition, Physical Activity and Obesity, tells msnbc.com. And about a third of those women had abandoned plans to exclusively breast-feed by the time they took their baby home from the hospital.

“To me, this isn’t about the individual women,” Perrine says. “This to me says we as a society are not supporting mothers to feed their infants the way they want to.”

The study analyzed data from about 1,500 U.S. women in a 2005-2007 study of infant feeding practices. Most of the women were 25- to 34 years-old, white and married and had some post-high school education. Their babies were all healthy.

The World Health Organization and the American Academy of Pediatrics recommend that mothers feed babies only breast milk (and medications or micronutrient supplements) for their first six months of life. “Breast milk is the best source of nutrition for young children and provides both short- and long-term health benefits,” including fewer infections and a lower risk of chronic conditions later in life, Perrine and her coauthors write.

The proportion of 6-month-old babies who’ve been exclusively breast-fed has increased in the United States, according to the CDC. In 2011, 14.8 percent were — up 4 percentage points from 2007, the first year the CDC issued a breast-feeding “report card.” That’s still well below the government’s target of 25.5 percent in the “Healthy People 2020” report.

In Perrine’s study, married moms with more than one child were more likely to exclusively breastfeed as long as they’d intended to. Moms who were obese, smoked or planned to breastfeed exclusively for at least seven months, which is longer than the minimum recommended by health experts, were less likely to meet their goal.

The researchers also found that six hospital practices helped predict whether new mothers would exclusively breast-feed as long as they’d planned:

• breast-feeding within one hour of birth
• no supplemental feedings with formula
• no pacifiers
• rooming in
• breast-feeding on demand
• information about breast-feeding support.

The most significant predictor was whether the hospital had supplemented breastfeeding with formula, which was reported by four out of 10 women in the study. Last summer, Perrine says, the CDC reported that about four out of five hospitals routinely gave formula to healthy breastfeeding newborns.

“I don’t think we know exactly why all of the hospitals are giving the formula,” says Perrine, whose study didn’t count those supplemental feedings as the end of exclusive breast-feeding. “I think it could be the weight issue. I think some nurses say let the mother sleep. It comes from a place of good intentions, but not everyone realizes how detrimental it can be to establishing breastfeeding.”

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