Researchers found that for each extra month women reported breastfeeding, their children performed slightly better on those exams - though not on tests of motor skills and memory.
"Given the size of the benefit, I think this should be helpful for women who are trying to make decisions about how long to breastfeed... because there are many factors that go into that decision," said Dr. Mandy Belfort, who led the study at Boston Children's Hospital.
"You have to weigh that against the time that it takes, maybe the time that it takes away from work and your other family duties."
She said the findings support recommendations from the American Academy of Pediatrics and other groups for exclusive breastfeeding up to six months of age, followed by a mix of breastfeeding and solid foods.
For their study, Belfort and her colleagues tracked 1,312 Massachusetts women who were recruited while pregnant in 1999 to 2002 and their babies.
Mothers reported if they had ever breastfed, and if so how old their child was when they stopped. The researchers then gave both women and their children standardized intelligence tests.
On language tests given at age three, children in the study scored an average of 103.7. Once the women's intelligence and other family factors including income were taken into account, the researchers found that each extra month of breastfeeding was tied to a 0.21-point improvement on the exam.
Children who were fed only breast milk for six months scored an average of three points higher on the language test than those who were never breastfed, Belfort and her colleagues write in JAMA Pediatrics.
For intelligence tests that included reading and writing given at age seven, average scores were 112.5 and each extra month of breastfeeding was linked to a 0.35-point improvement.
Those tests take 10 to 20 minutes to complete, and 100 is considered an average score across all children.
Belfort said a parent or teacher probably wouldn't notice a difference of a few points on a child's intelligence test.
"I think the importance is more on the level of the whole population or society," she told Reuters Health.
Image: Breastfeeding newborn, via Shutterstock