There’s medical answers to this question, though they vary from source to source.
But the one you need to know as a parent is this: Autism is the wall that separates your child from you.
The wall is different for everyone. Sometimes it’s their inability to hear you. To respond to you. To communicate with you. Worse, it’s a wall that you watched go up, brick by brick, unable to stop the mortar from drying, unable to stop the bricks from rising higher and higher until your child is all but gone.
Your baby was probably born completely normal. Healthy and fine. Sure, there were a few twinges of difference, maybe they had some quirks about how they ate, made certain rejections of nipples or bottles. For myself, I breast fed and my son had one position--I lay on my side and he lay next to me. Nothing else would work and he outright refused formula. If he did take it, it never stayed in his stomach long enough. I decided my son was set in his ways. I had no idea.
Maybe something happened, something unusual, like the baby making sounds incredibly early. Or physical milestones didn’t happen when they should. Or the baby does things slightly out of order.
My son used to make sounds, at three months he made a sound that sounded remarkably like Mama. He got the nickname “Bubba” because until he was six months, that’s pretty much the sound he made all day long. Around nine months he got to enjoying the sound of “dadah”. Then he was silent. It was a silence he’d maintain until he was nearly four years old.
He held himself up minutes after he was born. By two weeks he could lift his head and pick himself up slightly to look around. I was, of course, unspeakably proud. He could stand with a fingertip hold at three months and we took plenty of pictures of this veritable miracle. But I never saw him roll over. He just didn’t seem capable. One day, he had done it and only would do it if no one was looking. I thought this was just a quirk. He waited more months to army man crawl. He had no interest in crawling at all. He tried walking for a few hours at 9 months. Then he decided it wasn’t for him and didn’t bother with it until he was 15 months old. That was the beginning. There was a long road ahead.