The benefits of breastfeeding to both mother and child have been well-documented. Among other things, it can boost a child’s immunity and help protect moms from several different types of cancer. However, a new study by the American Sociological Review says breastfeeding may hurt mothers in an unexpected way – in the pocketbook.

According to the study, which looked at 1,313 first-time mothers in the US in their 20s or 30s, income among women who breastfed for six months or longer dropped sharply and stayed that way for five years. Mothers made about $5,000 less per year than they had before their babies were born.

One possible explanation is that moms are forced to reduce their hours in order to accommodate breastfeeding. In fact, the study’s author, Phyllis L.F. Rippeyoung, said she found it so difficult to juggle breastfeeding and work that she chose bottle feeding instead.

“I was a grad student at the time driving back and forth between teaching and classes, and my milk was drying up since I couldn’t drive and pump at the same time,” she said. “It was a very difficult thing, but I had to stop breastfeeding. If I’d continued I couldn’t have worked at the same time.”

A solution to the problem could include on-site daycare options at work. “If there were more ways in which women could combine breastfeeding with working you’d see less of this earnings decline,” said Rippeyoung.

“If there’s going to be a push for women to breastfeed then we need to take into account all of the costs,” she continued.