http://www.futurity.org/folate-pregnancy-autism-116048/
It's the same story it often is in biology - too little can be bad, too much can be bad:The researchers found that if a new mother has a very high level of folate right after giving birth—more than four times what is considered adequate—the risk that her child will develop an autism spectrum disorder doubles.
Further, very high levels of another vitamin, B12, are also potentially harmful, tripling the risk that a new mom’s offspring will develop an autism spectrum disorder. If both levels are extremely high, the risk that a child develops the disorder increases 17.6 times.
People need to be careful about supplementation - these are both common in pills:We have long known that a folate deficiency in pregnant mothers is detrimental to her child’s development. But what this tells us is that excessive amounts may also cause harm. We must aim for optimal levels of this important nutrient.