Mothers in the study, published in Pediatrics, who went back to work within six weeks were less likely than other women to start breastfeeding -- and when they did start, they were less likely to continue.
By contrast, mothers who stayed home for at least nine months, or even 13 weeks, were more likely to predominantly breastfeed their babies for three months or more.
Research has shown that breastfed babies have lower rates of a number of pediatric illnesses, including eczema, middle-ear infections, pneumonia and asthma.
"We would encourage all women to attempt to breastfeed and continue as long as they can," said study author Chinelo Ogbuanu at the Georgia Department of Health.
For example, she said, the more women breastfeed, the more milk they produce -- and when they are separated from their babies during the day, their milk supply may start to dwindle.
"No matter how effective a breast pump is, it's not as effective as an infant," Ogbuanu told Reuters Health.
She suggests that women try to take all their maternity leave at once, rather than breaking it up, and find ways to keep their baby close to the workplace during the day so they can breastfeed during work breaks. Failing that, regular pumping will ensure that their supply continues.