I'll sound like a right old hippy and pro-natural anti-drug birth nut, but that's because I am, - NOW.
Shedloads of oxytocin floods the woman's body to do a variety of things. To relax all the muscles to make birth easier, to keep her calm and in tune to move and be responsive during the birth, to keep the contractions effective, and to pass to her baby to keep the baby de-stressed and calm too.
If you imagine the tingle you get when having a head-massage, that is the kind of feeling you are 'supposed' to feel when having a baby, though not denying the pain aspect too.
This is a very sensitive time and easily disrupted. If the woman is scared, the baby will refuse to come out for a bit, as the adrenaline in her blood will signal the current time and place is a dangerous time to be born and survive. This is why I'm against the time limit of 10 days overdue. It can actually cause a very overdue baby.
During the birth process, fear and/or disruption can inhibit the necessary oxytocin production and slow down the birth, tire the woman to the extent that she needs more interference, change of venue, to have to talk to someone, drugs to further impede the oxytocin or override it, to worry about timeframes, to come completely out of the 'zone' she is supposed to be in.
I believe it is this disruption to birthing women on a wide scale that is responsible for the rise in simplex autism.
If a baby is born in such a context (one that signals the world they are about to be born into is dangerous) then to survive/be adaptive, they need to develop a more defensive set of senses.
This isn't simple, and a hard birth does not guarantee autism being triggered, in the same way that being born in the summer doesn't guarantee you will have brown eyes (though there is some research to shows that eye colour changes over the first couple of years in line with the environment and the light levels around the time of birth). If this is true then why not of autism tendency. Of course, if your whole family going far back forever had blue eyes then summer or not, you'll not be having brown as the possibility isn't even there.