Fat mamas and birth defects

Sandy at Junk Food Science did a fantastic job of taking the "omg fat wimmins are gonna have deformed baaaaaaaaybees" press release to task. What I'd like to do here is break that down into a bite-size information nugget to be cut and pasted when we're faced with that particular bit of hysteria:

Actually, if you look closely, the study found that there was only an association between fat mothers and neural tube defects (like spina bifida), not with any other defects--and, this was a correlation, which does not mean the defects were necessarily CAUSED by the mother being fat. It's well known that neural tube defects have a strong genetic component as well as being directly linked to folic acid deficiency; they can be mostly prevented by folic acid supplementation before and during pregnancy.

Interestingly enough, dieters tend to avoid foods rich in folic acid because they think they're fattening; a lot of our folic acid comes from enriched grain products like pasta, cereal, and bread, which are regarded as eeeeevil carbohydrates. So, a fat woman, who's more likely to be dieting, could be more likely to have folic acid deficiency, leading to neural tube defects. In that case, her FAT wouldn't be the problem; DIETING would be. Imagine that.