It turns out that head size is all the correlates to intelligence. Birth weight very weakly correlates to intelligence. Unless the birth weight is very low or very high. (This is taken from the book "What going on in there?" and all the other research that we've read.)

I heard yesterday on TV news (haven't scoured the web yet, but google news turns up plenty of hits for "autism mercury") that it's recently come to light that over the past decade or so, increasing rates of vaccination (most of which contain mercury compounds) have correlated with increasing rates of autism. There are other spins to put on the upward trend in autism, such as increased reporting as the disease became better-known (a la Rainman, for instance) or different diagnosis techniques. The CDC has been reluctant to mention the mercury correlation to the public because they don't want people to refuse all vaccinations for their children and thus create epidemics a few years later of preventable diseases. Oh, and pharmaceutical companies give lots of money to politicians. That too.

Mercury-containing vaccines have been outlawed in at least a few states, and I hear CA is considering that bandwagon.

This does not negate other studies that show that as babies got over 8lbs the effect starts to go away. eg. An 11 lb baby start sees a decrease in IQ.

As an unrelated note, one of many negative effects of a c-section (and I had one) is that women have a decreased success rate at breastfeeding. One reason for this is the milk tends to come in later. As we know, breastfeeding also contributes to an increase in IQ. A much stronger increase.

What we see possibly increasing c-sections as a choice of birth is the perceived convenience of being able to say I will have my baby on such and such day and time.

They also have higher rates of STD's which contribute to their much higher rate of premature babies.

I was 4lbs 11oz when I was born. In which direction does birth weight correlate?

I was born C-Section, because I refused to spend nine months upside down. Don't know quite what this does for your data...

Might we become a species that is born only by C-section?

If you're counting on natural / sexual selection, this would require a reversal of trends. Today "all the stupid people are breeding".

"I'd rather have a C-section than be in 'C' section"
- Old Video Only advertising department slogan.