Do I have to have an amnio if the ultrasound shows lots of findings of trisomy 18?

Diagnosis vs screen. Most maternal-fetal medicine or OB doctors. Sonographers spend time looking at the heart and other structures for abnormalities that can be associated with chromosomal or other problems. Details they see change with fetal age and position. Because there might be structures they cannot see, ultrasound is considered a screening test. To diagnose trisomy requires amniotic fluid or baby blood studies.
Decision making. The one and only purpose to undergo a screening or diagnostic test is to provide information that can help with making a decision. If ultrasound results already provide enough information for you to make a decision, it does not make sense to do amnio.
Depends. If the findings would determine whether the involved mother will choose to terminate or not, the reliability of the test matters. If the mom would accept a child with normal chromosomes but some identified problems, some form of definitive chromosome study is needed.Whether this is an amnio or a NIPP study is a matter of personal choice.