Women over 90 are twice as likely to suffer from dementia than men of that age group, according to a new study.

Researchers at the University of California, Irvine, found that, of 911 seniors over 90 years old, 27% of men and 45% of women, on average, had some form of dementia. What’s more, they discovered that between ages 90 and 100, dementia rates for men remained stable, while the rates among women jumped significantly-from 27% at age 90 to 70% at age 99.

The likelihood of developing dementia doubles every five years between ages 65 and 85, but it was previously unclear whether or not this trend continues after 90, say report authors. Researchers used data from the 90+ Study, one of the largest, most comprehensive studies of the very old, to draw their conclusions. The results of the study are published in the online version of the journal Neurology.