They found that four of every ten women with childhood migraines had added at least 22 pounds since age 18, compared to three of ten women who never had the throbbing headaches.
Migraines have been linked to obesity before, but the new results held up even after taking into account that kids plagued by headaches might have been heavier to begin with.
The new study, published in the journal Headache, is the first to tie childhood migraines to later weight gain, according to the researchers.
It's possible that the pain, at times accompanied by nausea and vomiting, makes women eat differently or alter their physical activity, but the study wasn't designed to answer that.
Still, it does hint that weight and migraines may somehow fuel each other, said Michelle A. Williams, of the University of Washington in Seattle, who led the work.
The results support earlier research showing a link between weight and headaches in children and young adults (although the link appears to weaken as women get older).
For instance, one 2009 study found that heavy children who lost weight while receiving treatment for headaches started having headaches much less often than their peers whose weight remained stable or increased.
"I would endorse the advice offered by the Centers for Disease Control and Prevention that promotes a lifestyle that includes healthy eating, regular physical activity, and avoidance of adult weight gain," said Williams, adding that more research is needed.