Carried out by researchers at Kaiser Permanente Northern California Division of Research in Oakland and the University of California Davis, the research is the first-ever large-scale study in the USA to look at a possible association between reproductive history and dementia risk in women.

The researchers analyzed self-reported data from 14,595 women age 40 to 55 and found a correlation between risk of dementia and the number of children, number of miscarriages, age at the time of first menstrual period, age at natural menopause, and reproductive period — the number of years between first menstrual period and menopause.

After taking into account mid- and late-life risk factors, such as body mass index and stroke history, the researchers found that the women in the study with three or more children had a 12% lower risk of dementia compared to women with one child.

After asking the women about miscarriage and menstrual history, the team found that each additional report of a miscarriage was associated with a 9% increased risk of dementia, compared to those women who reported no miscarriages, and that women who reported having their first menstrual period at age 16 or older were at 31% greater risk than those who reported having their first menstrual period at 13.

Those who experienced natural menopause at 45 or younger were at 28% greater dementia risk, compared to women who experience natural menopause after age 45, and women with reproductive periods of 21-30 years had a 33% higher dementia risk compared to women with reproductive periods of 38-44 years.

In a separate smaller study of 133 elderly British women, also presented at the conference, researchers at the University of California found that the number of months of pregnancy —  especially months spent in the first trimester — may also be a significant predictor of Alzheimer’s risk. The results showed that a woman who spent 12.5% more months pregnant than another, otherwise identical woman, had an approximately 20% lower risk of Alzheimer’s, with the team suggesting that the beneficial effect that pregnancy has on the immune system may be responsible.

According to Alzheimer’s Association 2018 Alzheimer’s Disease Facts and Figures, of the 5.5 million people age 65 or older with Alzheimer’s in the United States, 3.4 million are women and 2.0 million are men.

The AAIC 2018 takes places July 22-26 in Chicago, USA.