The study, conducted by the American Academy of Neurology, was recently published in the academic journal Neurology. It looked at more than 3,500 women ages 60 and older (but with a median age of 71) from South Korea and Greece. It found that by the time these women reached their advanced ages, the signs of Alzheimer’s had increased significantly mainly for those who had delivered five or more babies in their lifetime.
According to CNN, the study also found that even women who hadn’t experienced dementia but had given birth five or more times scored lower on cognitive tests than their peers who had mothered fewer children. Another important finding is that women who had one or two incomplete pregnancies had half the risk of developing Alzheimer’s, an unexpected phenomena that at first confused researchers.
One of the authors of the study, Dr. Ki Woong Kim, a neuropsychiatrist at Seoul National University, explained to CNN how surprised they originally were by the study’s findings. “Based on previous research, we expected that pregnancy with childbirth may be associated with the risk of Alzheimer’s disease,” he wrote. “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.”
There are several risk factors scientists have found connecting women to the degenerative disease, the greatest being old age. According to a study reported on by Scientific American, woman are living longer than men (the average lifespan for a US woman is 80.1, compared to the 73.4 years expected for US men). But that isn’t the only reason why two-thirds of all 5 million Alzheimer’s suffers are women.
The Alzheimer’s Association posited that “unique biological factors” cause women to be more likely to develop the disease. This includes the appearance of more amyloid plaques, a “sticky buildup which accumulates outside nerve cells, or neurons” that divides improperly in Alzheimer’s brains and instead creates a toxin to the brain’s neurons. The ApoE-4 gene in women can also double a female’s risk for the disease, whereas the gene in men only slightly elevates their risk.
But what Kim and other researchers have found is that hormones might be the variable between men and women that is truly causing such a large difference.
“In animals, early pregnancy was associated with improved cognition while late pregnancy and early postpartum with impaired cognition,” Kim told reporters. “In both animals and humans, estrogen is neuroprotective when it is modestly elevated while neurotoxic when extremely elevated.” Meaning that the more a women goes through the pregnancy process, the more she will experience these hormonal highs and lows.
Similarly, third-trimester pregnancies can yield “up to 40-fold higher” levels of estrogen when compared to even the highest peak of the menstrual cycle. But the levels drop dramatically once the baby is born.
Kim explained that the reason why researchers may not see these same results with women who miscarried or had incomplete pregnancies is because they never made it to these third-trimester peaks. “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,” he explained.
But some scientists who did not conduct the study say that it’s premature to draw these conclusions. James Hendrix, director of the global science initiatives for the Alzheimer’s Association, told CNN that “things like changes in the immune system, a healthy or poor diet, the level of exercise and just the stress of parenting five or more children may have an impact. Cultural differences could also play a role, as does access to health care” and not just hormones.
Hendrix said that the reason why women develop Alzheimer’s more frequently than men is still a question that science has yet to answer. “The takeaway for women is that we need more research to understand why. Women just don’t wake up one day and have Alzheimer’s. Brain health is a life course, and we need to understand how our brains are impacted with what we do early in life and throughout our lives,” he said.
