Time in the womb is obviously important for the development of the fetal brain. But pregnancy is also a time for changes in Mom's brain — changes that may prepare women to become better mothers.

These changes still are little-understood, concludes a review published in the December issue of the journal Current Directions in Psychological Science.

Pregnant women often complain about "pregnancy brain" or "mommy brain," a memory fog that seems to produce lost car keys and misplaced cell phones. One 2010 study suggested that high levels of sex hormones could be to blame for the frustrating lapses in concentration. But in many ways, the changes that happen in a mom-to-be's brain during pregnancy remain mysterious.

"Pregnancy is a critical period for central nervous system development in mothers," review author Laura Glynn, a psychologist at Chapman University in California, said in a statement. "Yet we know virtually nothing about it."

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Pregnant rats

It's difficult to examine the brains of pregnant women, and much of our modest knowledge comes instead from studies of rodents, Glynn said. Rats can be sacrificed and their brains studied in-depth. Studies have found that pregnancy causes rats' brains to form new olfactory, or smell-related, neurons. What's more, such pregnancy-related changes persist for the rest of the animals' lives. In rodents, at least, additional pregnancies seem to cause additional changes, so that the more litters a rat has, the more altered its brain will be.

It's not known whether women experience permanent changes as well; humans are very different from rats, after all. But according to Glynn, it's "extremely likely" that pregnancy permanently alters the human brain. The hormone flood that occurs during pregnancy dwarfs the hormonal changes that occur during other volatile times of life, such as adolescence, Glynn wrote. Just as teen hormones permanently alter brain structure and function, she said, it's likely pregnancy hormones do, too.

Your brain on pregnancy

In fact, recent research is revealing that fetuses have more effect on their mothers than previously realized. According to a 2004 study reviewed by Glynn, movements by the fetus after 20 weeks' gestation increase a mother's heart rate and her skin conductance, a measure of psychological arousal. These changes in Mom's physiology happen even when she can't physically feel the fetal movements. [5 Myths About Women's Bodies]

Fetuses share cells with their mothers, a strange phenomenon called microchimerism. Fetal cells pass through the placenta and lodge in a mother's body, where many remain quietly for years. One 2005 study found that in pregnant mice, these fetal cells hang out in the brain, especially in smell-related areas that are crucial for recognizing offspring. No one yet knows what, if anything, these fetal interlopers do there, or whether fetal cells in humans are similarly drawn to mothering-related areas of the brain.

Some of the brain changes during pregnancy may help mothers become more attuned to their infant when it is born, Glynn suggests. Fetal movements that tickle a woman's unconscious might prepare her to bond with her infant, for example. Likewise, changes in brain areas associated with emotion and memory could prime women for caregiving.

She said, "There may be a cost" — such as the perpetual fuzziness of "pregnancy brain" — "but the benefit is a more sensitive, effective mother."

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