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By Maggie Fox

Motherhood may affect a woman’s Alzheimer’s risk in unexpected ways, researchers reported Monday.

Women who had three or more children were less likely to develop dementia than women who had only one child, they found. And women who had miscarriages were more likely to develop dementia as well.

The researchers also found links between dementia risk and a woman's age at the onset of puberty or menopause.

The findings, presented at the annual Alzheimer’s Association conference in Chicago, might suggest that hormones affect a woman’s risk of developing the disease, said Heather Snyder, senior director of medical and scientific operations at the Alzheimer’s Association.

“It could be hormones,” Snyder told NBC News. Or it could have something to do with the immune system, which changes during pregnancy, or even the way a woman eats when she is pregnant.

“Or you can think about women with three children. They are multitasking a lot,” Snyder said. That could build up what are called brain reserves — extra reservoirs of brain function that have been shown to delay the symptoms of Alzheimer’s.

Paola Gilsanz of Kaiser Permanente in Oakland, California, and Rachel Whitmer at the University of California at Davis studied the cases of nearly 15,000 women ages 40 to 55 in the 1960s and 1970s.

Those who had three or more children were 12 percent less likely to develop dementia decades later, they found. The effect held even when Gilsanz and Whitmer took into account their weight and history of strokes, both of which affect dementia risk.

Miscarriages affected dementia risk also, they found. Every miscarriage a woman reported raised the risk of dementia by 9 percent.

In a separate study, a team that looked at 133 women in Britain found that those who spent more time being pregnant had a lower risk of Alzheimer’s than women who were otherwise very similar but spent less time being pregnant.

“We are intrigued by the possibility that pregnancy may reorganize the mother’s body in ways that could protect her against developing Alzheimer’s later in life,” Molly Fox, a behavioral scientist at the University of California at Los Angeles who led that study, said in a statement.

Snyder said the findings may point researchers to where they need to do more study. “Is it something about being pregnant?" she asked. "Is it something about stress of miscarriage?”

But there are other indications that hormones or general health may play a role.

Women who started puberty later had a higher risk of developing Alzheimer's. Women who started menstruating at age 16 or later had a 31 percent higher risk compared with those who started at 13, the average age.

And those who entered menopause early, before age 45, had a 28 percent higher risk.

It might not be that having more children is better, however. Last week, a team in South Korea found that women who had five or more children had a higher risk of Alzheimer’s.

“Estrogen levels double by the eighth week of pregnancy before climbing to up to 40 times the normal peak level,” Dr. Ki Woong-kim of Seoul National University, who led the study published in Neurology, said in a statement.

“If these results are confirmed in other populations, it is possible that these findings could lead to the development of hormone-based preventive strategies for Alzheimer’s disease based on the hormonal changes in the first trimester of pregnancy.”

Between 5 million and 6 million Americans have Alzheimer’s, according to the Alzheimer’s Association. There’s no cure, although drugs on the market can mask the symptoms for a while.

It’s important to understand the risk of Alzheimer’s in women, said Maria Carrillo, chief science officer at the Alzheimer’s Association.

“More women than men have Alzheimer’s disease or other dementias; almost two-thirds of Americans with Alzheimer’s are women,” Carrillo said.