Giving birth to five or more children could increase the risk of developing Alzheimer's disease, according to new findings. Incomplete pregnancies, on the other hand, were associated with a lower risk.

The study titled "Differential effects of completed and incomplete pregnancies on the risk of Alzheimer disease" was published in the journal Neurology on July 18.

According to the latest estimates, almost 5.7 million Americans of all ages are living with Alzheimer's dementia in 2018. Women make up two-thirds of the American population who have the disease.

The study involved more than 3,500 women from South Korea and Greece with an average age of 71. The participants provided information about their history of childbirth and also took diagnostic examinations to test their memory and thinking skills. The exam was taken after an average of 46 years since the first childbirth.

Overall, nearly 120 women developed Alzheimer's disease while nearly 900 women developed mild cognitive impairment, which is considered a precursor of the disease.

Among the 716 women who gave birth to five or more children, 59 had developed the disease. In other words, women who gave birth five or more times may be 70 percent more likely to develop Alzheimer's when they are older compared to those who had fewer births.

"Estrogen levels double by the eighth week of pregnancy before climbing to up to 40 times the normal peak level," said study author Dr. Ki Woong Kim, a neuropsychiatrist at Seoul National University in South Korea.

Going through five or more complete pregnancies would mean repeated and sudden exposure to the elevated production of estrogen and stress hormones. This could potentially affect brain power, which may explain the Alzheimer's link.

"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," he added.

But the most surprising finding concerned how abortion or miscarriage affected risk. Women who had an incomplete pregnancy had only half the risk of developing Alzheimer's compared to women who never had an incomplete pregnancy.

"Because most incomplete pregnancies occur in the first trimester of pregnancy, it is possible that the modestly raised levels of estrogen in the first trimester of pregnancy are within the optimal range for reducing the risk of Alzheimer's disease," Dr. Kim explained.

One of the limitations of the study is the lack of information regarding the timing and cause of the incomplete pregnancies. The researcher team did not take these factors into account.

And due to the use of self-reported data, the number of incomplete pregnancies may be inaccurate. It is possible that some women did not report their abortions or miscarriages. In some cases, women may have never realized that they had a miscarriage.