In the first study, published in the journal Neurology, Dr. Kurth and his colleagues looked at 27, 798 female health professionals ages 45 and older, including 3, 568 who had migraines, from the Women's Health Study.

CNN: Women's migraines linked to higher stroke, heart attack risk

Many companies now employ women guides, and 3 Sisters trainees have gone on to work for companies including One Seed Expeditions in Nepal and Chile, and Revive Treks, which operates in Nepal and the surrounding countries.

BBC: Breaking the glass ceiling of trekking