Are there ways to prevent having a child with downs syndrome?

Yes. Most often, down's syndrome, or trisomy 21, occurs in women over the age of 35 and increases with age. For people at risk, there are definitive diagnostic tests (cvs and amniocentesis) that can confirm or rule out a chromosomal abnormality. Also, most states offer comprehensive screening tests that inform patients whether they are at increased or decreased risk of carrying a down's syndrome baby.
Increased risk >35. Down syndrome is hereditary in approximately 1% of all instances. In the other 99% of cases Down syndrome is completely random and the only known factor that increases the risk is the age of the mother (over 35). So, having children before 35 years of age would lessen your risk. Also, children with Down Syndrome are WONDERFUL loving children so I would not worry about being able to prevent it.
Risks for any pregnc. The rate of ds births are lowest in the early 20's at about 1/2000 with a gradual increase in risks that hit 1/100 at 40 .There is always a baseline risk of about 4% of an unexpected outcome with any pregnancy.This must be added to specific age related risk. A visit with a genetic counselor can identify specific risk for any couple.Some screening tests can identify the problem early.