(CNN) — Whether children were breastfed as infants and for how long may have an impact on their test scores when they are adolescents, according to new research.

The report, published Monday in the journal Archives of Disease in Childhood, followed about 5,000 British children from their infancy in the early 2000s to their last year of high school, according to lead study author Dr. Reneé Pereyra-Elías, a doctoral student and researcher in the National Perinatal Epidemiology Unit at the University of Oxford.

The children were divided into groups based on how long they were breastfed: not at all, a few months, or for a year or more, according to the study. Researchers then compared the children’s results in the UK’s General Certificate of Secondary Education testing in their final years of secondary school.

What the study team found was that there was a modest improvement in test scores associated with being breastfed longer, Pereyra-Elías said.

Compared with those who never had breast milk, children who were breastfed for at least 12 months were 39% more likely to have a high pass for both math and English GCSE exams and were 25% less likely to fail the English exam.

But that does not mean that every family must breastfeed their child, Pereyra-Elías said.

It isn’t possible for every family to breastfeed, and those who don’t should not be shamed or feel guilty that they might be putting their children at a disadvantage, he said.

The analysis is careful and especially strong due to the size of its sample, said Dr. Kevin McConway, professor emeritus of applied statistics at the Open University in England.

McConway was not involved in the research.

“Though the results are certainly interesting, you have to bear in mind the limitations that inevitably arise in research using observational data from major cohort studies,” McConway added.

The link between breastfeeding and test scores

The fact that the study was observational means it followed people’s behavior rather than randomly assigning the behavior in question, McConway noted.

Consequently, the results only show a correlation between breastfeeding and test scores — not causation.

“It’s not possible to be certain about what’s causing what,” he said.

In the United Kingdom, mothers who have a higher socioeconomic position are more likely to breastfeed their children, and their children are more likely to do well in school, McConway said.

“That doesn’t mean that it’s the breastfeeding that causes the children to do well at school — obviously it could be some other aspect of the fact that their family is relatively well off,” he added.

It could be that something about breastfeeding causes children to be more likely to do well on their exams, but it could also be that another independent factor influences both the chances that the child will be breastfed and do well on their tests, McConway said.

The researchers tried to control for many factors that might influence their results, like the mother’s cognitive ability, but they couldn’t account for everything in an observational study, Pereyra-Elías said.

“There may be some confounding factors,” he said. “We did the best we could.”

The benefits of breastfeeding

The study showed test results as one of many possible benefits of breastfeeding, said Dr. Andrew Whitelaw, professor emeritus of neonatal medicine at the University of Bristol in England. Whitelaw was not involved in the research.

The difference this study showed was modest, Pereyra-Elías added, meaning that it does not make a big enough difference on the test scores that it should cause parents worry, Pereyra-Elías said.

The takeaway is that families in general should be encouraged to breastfeed because of multiple possible benefits, but that it still may not be best for each individual family, he said.

And more studies need to be done to confirm the findings — especially ones that account for the variables among families, Pereyra-Elías said.

“Even though these questions have been around for almost a century, we still do not have a definitive answer,” he said.

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