Hi Jenny! Welcome!
The triple screen is notoriously innacurate -- it has a very high false positive rate. And it is not a test with a definitive answer, it's a screening, which means that if the results are positive, you will need additional testing to confirm or deny the results. The additional testing involves Level II ultrasound(s) and an amnio. You should decide if you'd be willing to have an amnio should the results come back positive, and what you would do with the information should the news not be good. (For example, would you terminate the pregnancy if the baby has Down Syndrome?)
At age 20, the likelihood of a chromosomal defect is QUITE small.