Women use their brains differently at different times of the month, research suggests.
Emotion-controlling brain activity increased premenstrually
Brain scans revealed mental processes can change across the menstrual cycle.
Just before a period, at the time when some women experience premenstrual syndrome, activity in brain regions that help control emotions increased.
After menstruation the activity went down, a US team from New York told Proceedings of the National Academy of Sciences.
Although the researchers looked at 12 women with no outward menstrual mood changes, they say their findings could be important for understanding why some women have a particularly hard emotional time around menstruation.
PMS is believed to affect between one-third and one-half of women between 20-50 years of age.
Women with PMS may experience depression, irritability and a propensity towards outbursts of anger as well as physical symptoms such as cramps and bloating.
In the study, Dr Emily Stern of Cornell University, along with colleagues from the Rockefeller University, used MRI scans to monitor the brain activity patterns of women as they were asked to read words with negative, neutral or positive connotations.
The 12 women were asked to perform the same task premenstrually - one to five days before their period was due - and then postmenstrually - eight to 12 days after their period.
During the premenstrual phase the women showed much greater activity in frontal brain regions that help control emotions when they were reading the emotive words.
Postmenstrually, this increased brain activity had disappeared.
The researchers say it is possible that the brain changes might have allowed the women to maintain a consistent emotional state and compensate for the surging hormones that occur around menstruation, which some suggest are involved in PMS.
It might be that women who experience particularly severe emotional symptoms as part of PMS lose this control.
They plan to test this by looking at women with premenstrual dysphoric disorder (PMDD) - an extreme form of PMS.
They also suggested that doctors consider what phase of the menstrual cycle a woman is in when trying to interpret any brain scan results.
Chris Ryan, chief executive of the National Association for Premenstrual Syndrome, said: "This study, although of a small sample, adds to the compelling evidence that the menstrual cycle is a key factor affecting the psychological well-being of women of reproductive age.
"It confirms that the psychological health of women patients must be assessed in relationship to the menstrual cycle."