Prenatal screening includes blood tests and ultrasound examinations, which are done starting from 11 weeks of pregnancy which provide you a probability of having a baby with Down syndrome or other chromosomal abnormalities, structural abnormalities, risk of developing high blood pressure in the pregnancy, probability of having a small baby or an early delivery.
Screening tests cannot tell you with certainty whether or not your baby has one of these conditions, but only tells the chance of this.
To find out for sure, a Diagnostic test would be required