A massive study found a link between pregnancy and Alzheimer’s disease

A woman’s reproductive history may influence her risk for dementia later in life, new research suggests.

Scientists are desperate to understand what contributes to a person’s risk for developing Alzheimer’s disease and related dementias. Prominent theories have focused vascular disease like high blood pressure or the body’s immune system (paywall). But researchers have also noticed that roughly two-thirds of all Alzheimer’s patients are women, suggesting that gender may also factor into the disease.

The new research, presented at the 2018 Alzheimer’s Association International Conference in Chicago yesterday (July 23), suggests that, specifically, a woman’s pregnancies and her onset of menstruation and menopause may influence her risk for dementia.

Presented by scientists from the University of California at Davis and the Kaiser Permanente healthcare group, the study included 14,595 women between the ages of 40 and 55 who used the healthcare company’s coverage between 1964 and 1973. They were still using that coverage 32 to 44 years later, between 1996 and 2017, which provided the group with decades of data.

The team found that women who had three children or more were 12% less likely to develop dementia than their peers, even while controlling for other dementia risk factors. They also found that women whose periods started later in adolescence were 31% more likely to develop dementia than those who started menstruating sooner, and that women who stopped getting their periods before age 45 were 28% more likely to develop the disease than those who hit menopause later.

Why these relationships between reproductive history and dementia exist is unclear. The reason could be the women’s “cumulative exposure to estrogen across the life course,” Paula Gilsanz, a research at Kaiser Permanente Northern California and an author of the study, told the Washington Post (paywall). Women who are menstruating have higher levels of estrogen than those who are not, and pregnancy drastically increases a woman’s estrogen levels. In addition to higher levels of estrogen, a woman’s body goes through hundreds of other changes when a child is developing that could provide some kind of protective mechanism against dementia later in life.

Alternatively, the way that women care for themselves while pregnant could contribute to lowering their risk of dementia. During pregnancy, women are advised to stop drinking, smoking, and consuming caffeine for the protection of their developing child. Perhaps reducing these substances over the duration of at least three pregnancies—cumulatively more than two years of time, in most cases—could be protecting the women in the long run. Poor-quality, processed foods have also been labeled as a potential cause of Alzheimer’s disease, and women who are pregnant may be more vigilant about avoiding these foods in favor of more nutritious options.

The lower risk of dementia in women with at least three children may also have something to do the women’s lifestyles. “They are multitasking a lot,” Heather Snyder, the senior director of medical and scientific operations at the Alzheimer’s Association, told NBC news. Challenges like multitasking, planning and scheduling, attending school, and even socializing over a game may help build up the brain’s ability to resist cognitive decline later in life.

The researchers also found that women who reported having any number of miscarriages were 8% more likely to develop dementia than women who did not. However, women who have had miscarriages may have some underlying health risk that also happens to be a risk factor for developing dementia later.

Although this study was one of the largest on women and dementia, and fairly diverse—roughly a third of participants were non-white—it’s far from conclusive. Last week, another study found that having more than five children was linked to a higher risk of developing dementia. Whatever the relationship is between reproductive history and dementia, it is clearly complex, much like the disease itself.

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