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DEBBIE ELLIOTT, host:

More now from the department of sex differences. An item in the New York Times Magazine caught our eye this past week. It said that while men's brains are bigger, women have the same number of brain cells. They're just more tightly packed. Neuropsychiatrist LeaAnn Brizendine, who spoke with the Times, told me that male and female brains start going down separate paths very early on.

Ms. LEAANN BRIZENDINE (Neuropsychiatrist): Well, one thing we know for sure, Debbie, is that we all start out with a female-type brain in utero up until eight weeks of fetal life, when the tiny testicles and the male fetus start pumping out all that testosterone. It goes up and marinates the brain, changes it from female-type brain circuits into male brain circuits, increasing some areas, like the areas for sexual pursuit. And once we're born, we either have a female-type brain or a male-type brain.

ELLIOT: And how does that affect us when we're grown?

Ms. BRIZENDINE: Well, once we're grown, of course, then we start having the hormones that run our circuits. Right, the testosterone in the male starting in puberty starts to run the male brain circuits, and estrogen and progesterone in the female start to run the female brain circuits, getting us all ready for fertility and bearing children, pregnancy, breast-feeding and all the things that the female brain has to control.

ELLIOTT: Now, I saw you quoted in the New York Times, speaking of pregnancy, that the female brain shrinks about eight percent during pregnancy and doesn't return back to its normal size until about six months after delivery?

Ms. BRIZENDINE: Yes, Debbie, that's a surprising study that has found eight percent shrinkage even after you account for any increased water weight. And scientists don't know really why that happens except that the female brain is doing all kinds of rewiring during that period to get the mom ready to do maternal behavior.

And also, remember, the fetus is more like a parasite and it gets fed whatever it wants, and lots and lots of lipids and special fats exist inside the brain cells, so some scientists speculate that the fetus is sort of snacking on the mother's brain.

ELLIOTT: That explains what I used to describe as pregnancy brain.

Ms. BRIZENDINE: Yes, exactly.

ELLIOTT: Was I just making an excuse or was I really impaired because my brain was smaller when I was pregnant?

Ms. BRIZENDINE: No, Debbie, I think that your - remember, you've got 30 or 40 times the level of estrogen and progesterone coursing through your brain, getting you ready for the job at hand. But the good news is, at six months post-partum, the female brain comes back to its regular size and there are even some studies that show that we may be even smarter after the size returns and we have better connections.

ELLIOTT: LeaAnne Brizendine is a professor of neuropsychiatry at the University of California at San Francisco. Thank you.

Ms. BRIZENDINE: Thanks for having me, Debbie.

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