Q&A: I want to know how to determine the probability of my child having Down syndrome.

I am 31 years old and am 23 weeks pregnant with my first child. The results from my AFP test indicated a higher than normal risk for Down syndrome at 1:170. I visited with a genetic counselor, and let her know that we (myself and my husband--who already has 2 children) have no incidence of Down syndrome in the family.

I then went to a perinatologist who performed a level III ultrasound and found no markers for Down syndrome. He recalculated my risk at 1:340. I opted not to do the amnio. That said, I am finding it difficult not to worry. My regular OB indicated that an normal ultrasound was generally considered to be more reliable than the AFP (for Down syndrome). Is this true? How frequently do ultrasounds come back with no markers, only for the mother to deliver a baby with Down syndrome. Am I worrying needlessly?