When women are in the mood to make a baby . From New Scientist Print Edition. Subscribe and get 4 free issues.
. Rowan Hooper
IT MIGHT seem unlikely, but the mood swings many women experience every month may serve an evolutionary purpose – to get them pregnant.
Levels of sex hormones such as oestrogen and progesterone fluctuate throughout the monthly cycle. During the follicular phase at the start of the cycle, the egg is maturing and the body releases oestrogen, while during the luteal phase, when a fertilised egg might implant, progesterone is secreted.
To see how these might influence the brain, Jean-Claude Dreher and colleagues at the National Institute of Mental Health in Bethesda, Maryland, used functional MRI to look at how brain activity changes over the course of the month.
The team scanned the brains of 15 women at different stages in their menstrual cycle as they played a game with hypothetical prizes of money. During the follicular phase, the orbitofrontal cortex, an area of the brain associated with decision making and reward and emotion processing, and the amygdala, which also mediates emotional reactions, showed higher activity both when the women were anticipating a reward and when they received it (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0605569104).
This means the women were probably experiencing greater feelings of reward in response to the game during the first half of their menstrual cycles than during the second half, although they were not specifically asked to report this. “Our work only specifies the brain networks that are modulated by the menstrual cycle,” says Dreher.
It is unwise to speculate whether women also get more pleasure from activities such as sex, shopping or eating chocolate during the first half of their menstrual cycle, says Emily Stern of Cornell University’s Weill Medical College, New York, whose own work has shown how women use different parts of the brain at different stages of their menstrual cycle. “However, certain behaviours that are known to involve reward systems, such as drug addiction, might be enhanced during the follicular phase,” she says.
Indeed, previous experiments have shown that women report getting more pleasure from cocaine and amphetamine during the follicular phase compared to the luteal phase, says Dreher. He believes his findings may therefore help treat women with drug abuse problems or those with mood disorders.
Dreher also speculates that increased feelings of reward during the follicular phase – when a woman is ovulating and most likely to get pregnant – may have an evolutionary benefit. “It is interesting to note that from an evolutionary perspective, the increased availability, receptivity and desire that may occur during the ovulatory period has been thought to facilitate procreation,” he says.
From issue 2589 of New Scientist magazine, 03 February 2007, page 15