Story highlights More than 47 million people around the world are living with dementia

Research on a link to hormones and pregnancy is in the early stages

(CNN) Women who have given birth five or more times may be 70% more likely to develop Alzheimer's later in life than those who have fewer births, according to a new study of more than 3,500 women in South Korea and Greece. Even women without dementia who had given birth five or more times scored lower on a commonly used cognitive test than those with fewer children.

The study looked only at women older than 60; the average age of the women tested in both countries was 71.

The study also found that women who had experienced one or two incomplete pregnancies were much less likely to develop Alzheimer's than women who had never been pregnant. In fact, women who had an interrupted pregnancy had almost half the risk.

"Based on previous research, we expected that pregnancy with childbirth may be associated with the risk of Alzheimer's disease," study author Dr. Ki Woong Kim, a neuropsychiatrist at Seoul National University, wrote in an email. "However we were quite surprised that incomplete pregnancy was associated with the lower risk of Alzheimer's, which we did not expect at the beginning of our research."

Alzheimer's is a progressive mental deterioration of the brain that destroys memory and thinking skills until the person is unable to do even the simplest of tasks. Irreversible once it begins, the disease is thought to be caused by a buildup in the brain of beta amyloid plaques and neurofibrillary tangles called tau.

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