A lesser menstrual cycle over the lifespan as well as earlier menopause may explain why only some women are vulnerable to the risk of depression, researchers say.
The finding showed that longer duration of estrogen exposure from the start of menstruation until the onset of menopause was significantly associated with a reduced risk of depression during the transition to menopause and for up to 10 years post menopause.
The longer duration of birth control use was associated with a decreased risk of depression, but the number of pregnancies or incidence of breastfeeding had no association.
"Women are more vulnerable to depressive symptoms during and after the menopause transition because of fluctuating hormone changes," said JoAnn Pinkerton, Executive Director at the North American Menopause Society (NAMS) -- a US-based non-profit organisation.
"This study additionally found a higher risk for depression in those with earlier menopause, fewer menstrual cycles over lifespan or more frequent hot flashes," Pinkerton added.
Previous studies have suggested a role for reproductive hormones in causing an increased susceptibility to depression.
However, the new study focused on the effect of estradiol -- the predominant estrogen present during the reproductive years. It modulates the synthesis, availability and metabolism of serotonin -- a key neurotransmitter in depression.
While fluctuations of estradiol during the menopause transition are universal, the duration of exposure to estradiol throughout the adult years varies widely among women, the researchers said in the paper published in the journal Menopause.
"Women and their providers need to recognize symptoms of depression such as mood changes, loss of pleasure, changes in weight or sleep, fatigue, feeling worthless, being unable to make decisions, or feeling persistently sad and take appropriate action," Pinkerton said.
The team included more than 1,300 regularly menstruating pre-menopausal women aged 42 to 52 years for the study.