[Valpal1]

The risk of a firstborn with an autism spectrum disorder triples after a mother turns 35 and a father reaches 40.

The team found a 20% increase in the risk of autism with each 10-year increase in the parents' ages. Also, they found a couple's fourth child has half the risk of the first, regardless of the parents' ages.

Read more from Journal Sentinel: http://www.jsonline.com/news/health/37050669.html#ixzz2y8on18WJ

[Black Agnes]

Thus, environmental. Protection provided by previous pregnancies in some way.