Late 30s couple ttc for 1st time. no family history of autism/downs & both fit & healthy. what is % of risk for autism/birth defects?

Birth defects : About 1-2%. Put another way though, worst case scenario, 49/50 times all is normal.

If you'll be 39 at : Delivery, the chance to have a child diagnosed with Down syndrome at birth is 1 in 137 (a little < 1%); the chance to have a child diagnosed with any chromosome abnormality (including down syndrome) at birth is 1 in 83 (slightly > 1%). Autism risks ? with paternal age, low maternal folate (folic acid) levels esp. At conception-10 wks. & some prenatal infections like rubella or flu. Ask your OB @ screening.