You can add “healthier babies” and “stronger pocketbooks” to the list of reasons why paid sick days and paid family leave are essential to economic recovery for America’s working families:

The percentage of American mothers who breastfeed their babies has risen over the past decade, but it remains far below the rate public health officials would like to see. Newly published research provides one possible reason why.

It turns out all that healthy, nutritious milk comes at a surprisingly high cost.

A study of 1,313 American women who gave birth between 1980 and 1993 finds those who breastfed for six months or more suffered “more severe and more prolonged earnings losses” than mothers who breastfed for a shorter amount of time, or not at all.

“Our results suggest that breastfeeding, at least for six months or longer, is not free in an economic sense,” write sociologists Phyllis Rippeyoung and Mary Noonan. Their findings are published in the American Sociological Review.

According to the most recent government statistics, 74.6 percent of mothers report they breastfed their babies. But only 44.3 percent were still breastfeeding at six months, and 23.8 percent at one year, the minimum cutoff age recommended by the American Academy of Pediatrics.

In an attempt to find out why, Rippeyoung and Noonan studied data from the National Longitudinal Survey of Youth, an annual survey of a large group of Americans born between 1957 and 1964. They specifically looked at how childbirth and subsequent breast feeding impacted a woman’s earnings over the following years.

“We found that after childbirth, short-duration breastfeeders (those who stopped before the baby was 6 months old) and formula-feeders experienced similar earnings penalties,” the researchers write. “By contrast, women who breastfed for long durations experienced a much steeper decline in earned income over the first five years of their children’s lives.”

The cause of this gap is clear enough: “Long-duration breastfeeders are more likely to be non-employed in the years following childbirth, and they work fewer hours when they are employed.”

Learn why – and what can be done – in Miller-McCune »