First of all, you need to make sure you are creating this child with the right person. Take a good hard look at both your pedigrees. Is there anyone with autistic-like tendencies there? Any secret Thomas-the-Tank-Engine lovers? Genetics is one of the possible causes, you know. Assuming that every last one of your ancestors are free from any suspicion of secretly autistic traits, then take a look at the age of the male of your couple. It's a rare case where the woman seems off the hook, but if the male is an older father, it's thought that almost all the new weird genetic mutations that might lead to autism come from his side of the contribution. So you might want to trade him in for a younger model. The other factor you want to check both sides for is the presence of autoimmune disorders, which might play a role in autism. Any diabetes, asthma, arthritis, thyroid disease, lupus, MS, stuff like that? No? All set there? You are ready to get pregnant!
Now there are just a few rules you must follow when pregnant. First of all, don't get sick. Fever during pregnancy is thought to be a culprit. It might be good to isolate yourself all during pregnancy to avoid that. Next, look long and hard at medication you might take. Ask your doctor. Your doctor might say whatever you are taking is just hunky-dory, and then a few years later it's discovered that it isn't. But doctor's orders! So if you take any medication at all, you might want to go back a step and not get pregnant to start with. Avoid being overweight. Avoid getting pre-eclampsia. Avoid being stressed. Avoid getting pregnancy-induced diabetes, or any thyroid problems. Just to be safe, have a picture-perfect pregnancy in all ways.
Now---during the birth. Don't have any birth trauma or lack of oxygen. Don't have your baby prematurely. Hold them right away and let them know how welcome they are. Most people don't believe autism is caused by "refrigerator mothers" subconsciously rejecting their babies anymore, but you know how those things swing back and forth, so avoid ever thinking a single negative thought about the baby.
Now it gets tricky. There's the vaccine question. Science doesn't seem to back up that vaccines or mercury in vaccines cause a problem, but many mothers and blogs and celebrities think it does, so you'll have to decide on that. Make sure the baby doesn't get any infections soon after birth, viral or otherwise. Some people think lately autism is caused by an over-clean environment not teaching the body some early immune responses, so be reasonably casual about germs, but of course, that's subject to change at any point, and if it's later decided dirt causes autism, you'll never forgive yourself. Lack of vitamin D is one theory, so live in a sunny climate. Too much rain could be a problem, so if you get hit with some long rainy spells, move. There's the whole possible diet connection, with lactose and gluten being suspected as problems. Who knows, but why not just never serve any of them to be sure? Early TV could be a problem, so get rid of your set.
And of course, if your child STILL is stubborn enough to show even the slightest autistic trait, you want to nip it in the bud. Have your child screened for autism starting at birth, probably every week will be enough. If you see the slightest sign of it, start ABA about 100 hours a week immediately, until your kid is so normal they could be a model for normalness.
And you know of course this is all very tongue in cheek. I've just been reflecting a lot lately on how the almost daily new ideas about what causes autism must put through the heads of someone determined to do all they can to give their kids a good head start, a nice autism-free life. And the moral is, of course, you can't do that. You could do everything possible known right now, which would result in some crazy doings, and in a few years, it could be determined that everything you thought right was wrong. We just don't know what causes autism. There probably isn't any one thing that causes it. So do what you feel is best. Do what you yourself decide is healthy and reasonable to do. Listen to a good mainstream OB/GYN or pediatrician. Use your own judgement. Don't listen to what bloggers say, including me.
And if, all else failing, you do end up with an autistic child, I'm here to say it's not the end of the world. You aren't a terrible person for somehow not being able to prevent that happening. You are a parent that like many parents from the beginning of time, were dealt a tough hand, but you will deal with it. Your child may not take you to Holland, but they will give you moments of extreme joy along with the hard times. Congratulations on your child, autistic or not.