The brain is still a big scientific mystery, and the effects of alcohol on a developing brain are even murkier.
Scientists say alcohol alters how individual brain cells operate and how entire regions of the brain function.
It reduces the density of grey and white matter and shrinks the brain itself. In very extreme cases, the brain looks almost mushy.
As pediatrician and FASD expert Sterling Clarren likes to say, "The big, important message is that alcohol does not affect a small piece of the brain. It affects everything.
Alcohol is a teratogen, a substance that causes birth defects.
TERATO: from the Greek meaning monster
GEN: from the Greek meaning make
Alcohol can shrink the corpus callosum, a thick band of fibres in the brain's core that connects the right and left himispheres, the logical and emotional halves of the brain.
If your corpus callosum is stunted, it's difficult for the two sides of the brain to talk to each other, resulting in problems storing and retrieving information, paying attention and problem-solving. Some people with severe FASD don't have a corpus callosum at all.
Alcohol can shrink the cerebellum, which is in charge of processing inputs from other areas of the brain to co-ordinate motor and thinking skills. If it gets damaged, fine motor skills can be ruined.
Individual brain cells can be mangled by alcohol, damaging everything from the regulation of gene expression, to the way cells interact, to the growth and survival of neural stem cells -- the basic regenerating cells of the brain.
The glial cells -- the security blankets that protect and feed each neuron -- can also be profoundly affected.
There is no known safe level of alcohol a woman can drink while pregnant without harming her baby.
Binge drinking or chronic alcoholism present the bigger risk.
But women who drink only occasionally could also damage their babies, depending on the development stage of the fetus and other factors such as nutrition, stress, smoking, how fast a woman metabolizes alcohol and old-fashioned genetics.
Not every child whose mother drank during pregnancy will suffer brain defects. Half emerge unscathed.
Genetics play a huge and mysterious part in determining which child will be most affected by alcohol in utero.
Even among twins, one might have a genetic tick that modulates the effect of alcohol while the other will suffer brain damage.
The worst effects occur during the first trimester. Organs are starting to form, so adding an alcohol "insult," as researchers call it, can cause congenital heart problems, kidney damage and brain defects.
It's also in the first trimester that the classic FASD face forms.
In the second and third trimester, alcohol will affect how the brain matures, especially its wiring.
Sources: Dr. Albert Chudley, and various academic papers, including Foetal Alcohol Spectrum Disorders and Alterations in Brain and Behaviour by Consuelo Guerri, Alissa Bazinet and Edward Riley (Alcohol and Alcoholism, 2009)