This means that the blood test has determined that your pregnancy has the same risk or higher as that of a 35-year old of being affected by Down's Syndrome. It is not diagnostic. This test is a screening test and is intended to delineate which patients should be offered further testing.
The most common reason for a positive result is the dates of the pregnancy are inaccurate. In this case, ultrasound-directed correction of the dates will typically yield a correct negative result. In the case where the dates are correct, you will be offered counseling and an explanation of the result with a perinatologist. You will be offered a level two ultrasound looking for features seen in Down's syndrome. These are by no means diagnostic. Amniocentesis is the gold standard for diagnosis. This involves placing a needle into the uterus under ultrasound guidance and withdrawing amniotic fluid which can then be tested.