Downs syndrome is caused when the chromosomes in the egg (it's almost always the egg) don't copy right and it adds an extra chromosome. This is called Trisomy because there are three chromosomes instead of the 2 that should be there. This is the only thing that can cause down syndrome- the extra chromosome. The older a woman is, the higher her risk is to have a baby with down syndrome. I also believe your risk is increased if you have already conceived a child with down syndrome. Having Schizophrenia shouldn't influence the risk factor so you should have the same chances as any other female your age.
Can someone with Schizophrenia have a child with down sydrome...yes..does Schizophrenia cause it NO.