I've just seen your updates OP.
First off, the combined screening is not a diagnostic test. NIPT isn't either, but is much more accurate.
Second off, all pregnancies are different. Complications arise even if you haven't had any before, some examples would be pre-eclampsia and gestational diabetes. There are things that increase your risk of them, but your risk is never zero. ANYONE can develop these things.
Not to mention other issues such as placenta previa, hypertension, pre-term labour, issues with antibodies, DVT, cholestasis.
Attending your appointments will increase the likelihood that these issues are spotted and dealt with, if they do arise, before it's too late.