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.
The fear of spiders is one of the quickest phobias to treat, with success usually achieved over a few hours in a single day.
The problem is that people tend to relapse over time.
Dr Graham's work could lead to psychologists paying more attention to the timing of treatment or to women being given estrogen supplements before treatment.
She hopes to extend her study to more difficult conditions like post-traumatic stress disorder and social phobia.
The British mental health charity MQ has awarded her more than $300,000 over three years to research her concept, which it describes as "elegant and groundbreaking".
Although women are twice as likely than men to suffer anxiety, research has traditionally focused on male subjects, says Dr Graham of the University of New South Wales.
"This means women might not be receiving the most appropriate treatment for a whole range of disorders.
"We need to start looking at sex differences and the influence of sex hormones."
In her spider project, clinically anxious women with a phobia will be gradually exposed to the creatures. In a matter of hours most will be able to pick up a spider with their bare hands.
The women's estradiol level at the time of treatment will be checked.
"We expect women with low levels at the time of their treatment are more likely to relapse after six months," says Dr Graham.