Patients receiving a positive screening result for Down syndrome should understand that the likelihood of the fetus actually having Down syndrome (Positive Predictive Value) is generally below 90%. For individuals aged 35 and older with a positive screening result for Down syndrome and Trisomy 18, the estimated risks are 87% and 63% respectively. Conversely, a negative screening result indicates a high probability of a normal fetus (>99.9%), yet it does not entirely exclude the presence of Down syndrome.