The stage of a woman's menstrual cycle may have a major impact on how well she responds to treatment for anxiety, says an Australian psychologist.
Dr Bronwyn Graham is testing her theory on women who are terrified of spiders.
They will be given exposure treatment, a form of cognitive behavioural therapy that helps people conquer their fears.
Dr Graham believes the benefits will last longest for those who have a high level of estradiol, a type of estrogen, on the day of treatment.
Estradiol levels are lowest just before and during menstruation. They are also low among women on the birth-control Pill.