And recent studies offer that hormone replacement therapy protects a woman's brain if it is given at the right time.

The findings could help explain why women make up nearly two-thirds of people in the U.S. with Alzheimer's.

One of the studies looked at nearly 15,000 women in California. And it found an association between a woman's reproductive history and her risk of memory problems later in life.

Studies showed the risk of dementia for women who had three or more children was 12 percent lower than the risk for women who had one child and women who began to menstruate earlier and went through menopause later were less likely to develop dementia. Menopause at age 45 or younger seemed to increase the risk by 28 percent.

Another study of 133 elderly women in the U.K. found that the more months of pregnancy they experienced during their lives, the lower their risk of developing Alzheimer's.

The findings all suggest that female sex hormones — which rise at puberty and during pregnancy, then fall at menopause — are somehow affecting a woman's risk of developing Alzheimer's and other forms of dementia. The results also suggest that greater exposure to these hormones, through more pregnancies or more reproductive years, can reduce a woman's risk.

Rapid changes in hormone levels present a problem.

Women who start hormone therapy between 50 and 54 are less likely to have trouble with thinking and memory.

Scientists have long suspected that sex hormones such as estrogen and progesterone play a role in Alzheimer's.

One way for women to minimize the dramatic hormonal changes that occur at menopause is to use hormone replacement therapy.