26 Weeks

One for the reference library - from Dr Leonard Sax's Montgomery Center for Research in Child and Adolescent Development:

Sex differences in the brain begin in the womb. About midway through pregnancy, the testicles of a developing baby boy start churning out testosterone in huge quantities, comparable to what an adult man produces. These sex hormones bind to brain tissue and begin to transform it. Between 18 and 26 weeks gestation, the developing brain is permanently and irreversibly transformed. Israeli scientists Reuwen and Anat Achiron have found that if you do a regular ultrasound examination when a woman is 26 weeks pregnant, you can distinguish a female brain from a male brain. At 26 weeks!

Source: Reuwen Achiron, Shlomo Lipitz, & Anat Achiron. Sex-related differences in the development of the human fetal corpus callosum: in utero ultrasonographic study. Prenatal Diagnosis, 2001, 21:116-120.

This in utero study confirmed the findings of a previous anatomical study in which investigators examined the brains of babies which had died before birth.

See: M. de Lacoste, R. Holloway, and D. Woodward, "Sex differences in the fetal human corpus callosum," Human Neurobiology, 1986, 5(2):93-6.

Structural sex differences in the human brain

Take a slice of the brain from somebody who's just died. Shave the slice very thin with a microtome (an instrument like a carrot peeler). Look at that translucent peel of brain tissue under the microscope. One group based primarily at the University of Lausanne in Switzerland has specialized in doing this work for more than ten years. They have employed computerized quantitative cytological techniques, using computer algorithms to measure the volume of individual nerve cells and the number of connections made by individual nerve cells, in specific areas of the brain. They have found that

"fundamental gender differences exist in the structure of the human cerebral cortex."

Source: Theodore Rabinowicz, Jean MacDonald-Comber Petetot, Peter Gartside, David Sheyn, Tony Sheyn, & Gabrielle de Courten-Myers, "Structure of the Cerebral Cortex in Men and Women," Journal of Neuropathology and Experimental Neurology, January 2002, 61(1):46-57. The quotation comes from page 52 of this paper.

It is not the case that every area of a girl's brain differs from every area of a boy's brain. The Lausanne group studied cells in the cerebral cortex, the most advanced part of the brain. Specifically, they studied cells in Layer IV of the cerebral cortex, that part of the cortex which receives input from other cells.

One research team recently compared brain tissue from the brains of young girls and young boys. They found that sex differences in the structure of the brain were obvious, even in babies -- especially in babies. The differences in the photomicrographs of the brain tissue are so dramatic that they are readily visible to the naked eye.

Source: María Elena Cordero, Carlos Valenzuela, Rafael Torres, Angel Rodriguez, "Sexual dimorphism in number and proportion of neurons in the human median raphe nucleus," Developmental Brain Research, 124:43-52, 2000.