Brain size, changes caused

Many chemicals cause a decrease in brain size during fetal development. Changes in brain size can be caused in various ways. A decreased brain weight may be a consequence of general undernutrition caused by effects on placental functioning. Examples of other possible mechanisms are inhibition of cell multiplication or cell growth, increased cell death, disrupted trophic effects induced by changes in cell maturation and neurochemistry. 

Since we already know that changes in the fetal environment can cause severe changes in the development of the fetal brain and nervous system, one question that pops up is whether mere brain size can explain IQ differences—brain size either determined by genes alone or brain size determined by gene—environment interactions during gestation. 

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