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Last summer, New York City Mayor Michael Bloomberg said he was going to tell hospitals to keep infant formula locked up in cabinets to encourage new mothers to breast-feed. He might want to reconsider: A recent study shows that limited early formula supplementation might help some moms breast-feed longer.

Doctors took 40 babies who were between 1 and 2 days old, exclusively breast-feeding, and had lost between 5 and 10 percent of their birth weight. Twenty of those babies were given only breast milk. The other 20 were given a small dose of formula via a syringe (to avoid nipple confusion) after they breast-fed – not enough to make them full and possibly reject breast milk at their next feeding. These formula moms only supplemented their breast milk until their mature milk came in.

The results were that moms who had supplemented their breast milk in the first days were less likely to use formula down the road than the mothers who were exclusively breast-feeding during those difficult early hours. At three months, 95 percent of the babies who had received formula in those first few days were still getting some breast milk, compared with only 69 percent of the babies that had received no formula. And as an added benefit, the babies who were given formula lost slightly less weight in the short run. The study was conducted by the University of California, San Francisco.

It’s difficult to know exactly why that tiny bit of formula in the early days made the difference, but one speculation is that it eased anxiety about the babies gaining weight. Since many moms stop breast-feeding because they’re concerned that their children aren’t getting enough food, getting that limited boost from formula in the early days gave them the assurance to keep going with breast-feeding, the study’s authors hypothesized.

Forty women is a small sample, and as the authors point out, their sample size was mostly white and Asian, and more educated than the general population. And it doesn’t answer all the questions about early use of formula. But this is at least a rare infant study that was able to use a randomized sample.