Women whose menstruation starts later and those who enter menopause early may have a greater risk of developing dementia, say researchers.

The findings showed that women who had their first menstrual cycle at age 16 or older had a 23 per cent greater risk of dementia than women who had their first menstrual cycle at age 13. Women who went through natural menopause before age 47 had a 19 per cent greater risk of dementia than women who went through menopause at age 47 or older.

In addition, women who had hysterectomy -- surgery to remove all or part of the uterus -- had an eight per cent greater risk of dementia than those who did not, according to the study, published in the journal Neurology. "Oestrogen levels can go up and down throughout a woman's lifetime. Our results show that less exposure to oestrogen over the course of a lifetime is linked to an increased risk of dementia," said Paola Gilsanz, Researcher at Kaiser Permanente - a US-based healthcare company.

For the study, the researchers involved 6,137 women among which 42 per cent later developed dementia. "Since women are 50 per cent more likely to develop dementia over their lifetimes than men, it's important to study any risk factors that are specific to women that could eventually lead us to potential points of intervention," Gilsanz suggested.