The research, involving 30,000 women each of whom had produced five or more children, has shown that those with a gap of less than 12 month were 5.2 times more likely to develop the advanced ductal breast cancer than women who had a gap of three or more years.
Although it is unclear that why the risk increases, researchers believe hormones might be involved.
Alternatively, other risk factors may have influenced the results, including being overweight, and whether or not the mother chooses to breast-feed.
"Women who had their first two births close together should not be worried by these findings because the study's results are not conclusive," the Telegraph quoted Josephine Querido, senior science information officer at Cancer Research UK, as saying.
"The researchers looked at a very specific group of women - those who had a specific type of breast cancer, who had advanced tumors, who were under 50, and who had at least five children. In studies like this, dividing the group of people you're looking at into lots of smaller groups makes it likely that you'll find a positive result in one of the subgroups just by chance.
"For all women, it's important to go to the GP if they spot any unusual changes in their breast, and to go for screening when invited," Querido added.
The study appears in the British Journal of Cancer.