For the study, published in JAMA Pediatrics, researchers at Harvard Medical School enrolled 1, 312 pregnant women. The researchers assessed breast-feeding at 6 and 12 months and followed the mothers and children for seven years.

For each additional month a baby was breast-fed, verbal ability was higher at age 3, and verbal and nonverbal I.Q. scores were higher at age 7, the study concluded. The researchers accounted for factors like the mothers’ intelligence and employment, home environment and child care.

“One of the theories as to why breast-fed children tend to have better cognitive development is there are nutrients in breast milk that benefit the baby’s developing brain,” said Dr. Mandy Brown Belfort, a neonatologist at Boston Children’s Hospital and lead author of the study.

“Our results support policies that allow women to continue breast-feeding through a year of their child’s life to optimize brain development,” Dr. Belfort said. “On the individual mom level, it can be a really difficult decision, especially as they go back to work. Now we see how much extra they gain with each additional month they continue to breast-feed by pumping at work.”